ANIMA WG M. Pritikin
Internet-Draft Cisco
Intended status: Standards Track M. Richardson
Expires: January 22, 2020 Sandelman
M. Behringer
S. Bjarnason
Arbor Networks
K. Watsen
Watsen Networks
July 22, 2019
Bootstrapping Remote Secure Key Infrastructures (BRSKI)draft-ietf-anima-bootstrapping-keyinfra-23
Abstract
This document specifies automated bootstrapping of an Autonomic
Control Plane. To do this a remote secure key infrastructure (BRSKI)
is created using manufacturer installed X.509 certificate, in
combination with a manufacturer's authorizing service, both online
and offline. Bootstrapping a new device can occur using a routable
address and a cloud service, or using only link-local connectivity,
or on limited/disconnected networks. Support for lower security
models, including devices with minimal identity, is described for
legacy reasons but not encouraged. Bootstrapping is complete when
the cryptographic identity of the new key infrastructure is
successfully deployed to the device but the established secure
connection can be used to deploy a locally issued certificate to the
device as well.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 14, 2020.
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deployment. Section Section 9 explains the details applicability for
this the ACP usage.
This document describes how pledges discover (or be discovered by) an
element of the network domain to which the pledge belongs to perform
the bootstrap. This element (device) is called the registrar.
Before any other operation, pledge and registrar need to establish
mutual trust:
1. Registrar authenticating the pledge: "Who is this device? What
is its identity?"
2. Registrar authorizing the pledge: "Is it mine? Do I want it?
What are the chances it has been compromised?"
3. Pledge authenticating the registrar: "What is this registrar's
identity?"
4. Pledge authorizing the registrar: "Should I join it?"
This document details protocols and messages to answer the above
questions. It uses a TLS connection and an PKIX (X.509v3)
certificate (an IEEE 802.1AR [IDevID] LDevID) of the pledge to answer
points 1 and 2. It uses a new artifact called a "voucher" that the
registrar receives from a "Manufacturer Authorized Signing Authority"
and passes to the pledge to answer points 3 and 4.
A proxy provides very limited connectivity between the pledge and the
registrar.
The syntactic details of vouchers are described in detail in
[RFC8366]. This document details automated protocol mechanisms to
obtain vouchers, including the definition of a 'voucher-request'
message that is a minor extension to the voucher format (see
Section 3) defined by [RFC8366].
BRSKI results in the pledge storing an X.509 root certificate
sufficient for verifying the registrar identity. In the process a
TLS connection is established that can be directly used for
Enrollment over Secure Transport (EST). In effect BRSKI provides an
automated mechanism for the "Bootstrap Distribution of CA
Certificates" described in [RFC7030] Section 4.1.1 wherein the pledge
"MUST [...] engage a human user to authorize the CA certificate using
out-of-band" information". With BRSKI the pledge now can automate
this process using the voucher. Integration with a complete EST
enrollment is optional but trivial.
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BRSKI is agile enough to support bootstrapping alternative key
infrastructures, such as a symmetric key solutions, but no such
system is described in this document.
1.1. Prior Bootstrapping Approaches
To literally "pull yourself up by the bootstraps" is an impossible
action. Similarly the secure establishment of a key infrastructure
without external help is also an impossibility. Today it is commonly
accepted that the initial connections between nodes are insecure,
until key distribution is complete, or that domain-specific keying
material (often pre-shared keys, including mechanisms like SIM cards)
is pre-provisioned on each new device in a costly and non-scalable
manner. Existing automated mechanisms are known as non-secured
'Trust on First Use' (TOFU) [RFC7435], 'resurrecting duckling'
[Stajano99theresurrecting] or 'pre-staging'.
Another prior approach has been to try and minimize user actions
during bootstrapping, but not eliminate all user-actions. The
original EST protocol [RFC7030] does reduce user actions during
bootstrap but does not provide solutions for how the following
protocol steps can be made autonomic (not involving user actions):
o using the Implicit Trust Anchor [RFC7030] database to authenticate
an owner specific service (not an autonomic solution because the
URL must be securely distributed),
o engaging a human user to authorize the CA certificate using out-
of-band data (not an autonomic solution because the human user is
involved),
o using a configured Explicit TA database (not an autonomic solution
because the distribution of an explicit TA database is not
autonomic),
o and using a Certificate-Less TLS mutual authentication method (not
an autonomic solution because the distribution of symmetric key
material is not autonomic).
These "touch" methods do not meet the requirements for zero-touch.
There are "call home" technologies where the pledge first establishes
a connection to a well known manufacturer service using a common
client-server authentication model. After mutual authentication,
appropriate credentials to authenticate the target domain are
transfered to the pledge. This creates serveral problems and
limitations:
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o the pledge requires realtime connectivity to the manufacturer
service,
o the domain identity is exposed to the manufacturer service (this
is a privacy concern),
o the manufacturer is responsible for making the authorization
decisions (this is a liability concern),
BRSKI addresses these issues by defining extensions to the EST
protocol for the automated distribution of vouchers.
1.2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
The following terms are defined for clarity:
domainID: The domain IDentity is the 160-bit SHA-1 hash of the BIT
STRING of the subjectPublicKey of the pinned-domain-cert leaf,
i.e. the Registrars' certificate. This is consistent with the
subject key identifier (Section 4.2.1.2 [RFC5280]).
drop ship: The physical distribution of equipment containing the
"factory default" configuration to a final destination. In zero-
touch scenarios there is no staging or pre-configuration during
drop-ship.
imprint: The process where a device obtains the cryptographic key
material to identify and trust future interactions with a network.
This term is taken from Konrad Lorenz's work in biology with new
ducklings: during a critical period, the duckling would assume
that anything that looks like a mother duck is in fact their
mother. An equivalent for a device is to obtain the fingerprint
of the network's root certification authority certificate. A
device that imprints on an attacker suffers a similar fate to a
duckling that imprints on a hungry wolf. Securely imprinting is a
primary focus of this document [imprinting]. The analogy to
Lorenz's work was first noted in [Stajano99theresurrecting].
enrollment: The process where a device presents key material to a
network and acquires a network specific identity. For example
when a certificate signing request is presented to a certification
authority and a certificate is obtained in response.
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Pledge: The prospective device, which has an identity installed at
the factory.
Voucher: A signed artifact from the MASA that indicates to a pledge
the cryptographic identity of the registrar it should trust.
There are different types of vouchers depending on how that trust
is asserted. Multiple voucher types are defined in [RFC8366]
Domain: The set of entities that share a common local trust anchor.
This includes the proxy, registrar, Domain Certificate Authority,
Management components and any existing entity that is already a
member of the domain.
Domain CA: The domain Certification Authority (CA) provides
certification functionalities to the domain. At a minimum it
provides certification functionalities to a registrar and manages
the private key that defines the domain. Optionally, it certifies
all elements.
Join Registrar (and Coordinator): A representative of the domain
that is configured, perhaps autonomically, to decide whether a new
device is allowed to join the domain. The administrator of the
domain interfaces with a "join registrar (and coordinator)" to
control this process. Typically a join registrar is "inside" its
domain. For simplicity this document often refers to this as just
"registrar". Within [I-D.ietf-anima-reference-model] this is
refered to as the "join registrar autonomic service agent". Other
communities use the abbreviation "JRC".
(Public) Key Infrastructure: The collection of systems and processes
that sustain the activities of a public key system. The registrar
acts as an [RFC5280] and [RFC5272] (see section 7) "Registration
Authority".
Join Proxy: A domain entity that helps the pledge join the domain.
A join proxy facilitates communication for devices that find
themselves in an environment where they are not provided
connectivity until after they are validated as members of the
domain. For simplicity this document sometimes uses the term of
'proxy' to indicate the join proxy. The pledge is unaware that
they are communicating with a proxy rather than directly with a
registrar.
Circuit Proxy: A stateful implementation of the join proxy. This is
the assumed type of proxy.
IPIP Proxy: A stateless proxy alternative.
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MASA Service: A third-party Manufacturer Authorized Signing
Authority (MASA) service on the global Internet. The MASA signs
vouchers. It also provides a repository for audit log information
of privacy protected bootstrapping events. It does not track
ownership.
Ownership Tracker: An Ownership Tracker service on the global
internet. The Ownership Tracker uses business processes to
accurately track ownership of all devices shipped against domains
that have purchased them. Although optional, this component
allows vendors to provide additional value in cases where their
sales and distribution channels allow for accurately tracking of
such ownership. Ownership tracking information is indicated in
vouchers as described in [RFC8366]
IDevID: An Initial Device Identity X.509 certificate installed by
the vendor on new equipment.
TOFU: Trust on First Use. Used similarly to [RFC7435]. This is
where a pledge device makes no security decisions but rather
simply trusts the first registrar it is contacted by. This is
also known as the "resurrecting duckling" model.
nonced: a voucher (or request) that contains a nonce (the normal
case).
nonceless: a voucher (or request) that does not contain a nonce,
relying upon accurate clocks for expiration, or which does not
expire.
manufacturer: the term manufacturer is used throughout this document
to be the entity that created the device. This is typically the
"original equipment manufacturer" or OEM, but in more complex
situations it could be a "value added retailer" (VAR), or possibly
even a systems integrator. In general, it a goal of BRSKI to
eliminate small distinctions between different sales channels.
The reason for this is that it permits a single device, with a
uniform firmware load, to be shipped directly to all customers.
This eliminates costs for the manufacturer. This also reduces the
number of products supported in the field increasing the chance
that firmware will be more up to date.
ANI: The Autonomic Network Infrastructure as defined by
[I-D.ietf-anima-reference-model]. This document details specific
requirements for pledges, proxies and registrars when they are
part of an ANI.
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offline: When an architectural component cannot perform realtime
communications with a peer, either due to network connectivity or
because the peer is turned off, the operation is said to be
occurring offline.
1.3. Scope of solution1.3.1. Support environment
This solution (BRSKI) can support large router platforms with multi-
gigabit inter-connections, mounted in controlled access data centers.
But this solution is not exclusive to large equipment: it is intended
to scale to thousands of devices located in hostile environments,
such as ISP provided CPE devices which are drop-shipped to the end
user. The situation where an order is fulfilled from distributed
warehouse from a common stock and shipped directly to the target
location at the request of a domain owner is explicitly supported.
That stock ("SKU") could be provided to a number of potential domain
owners, and the eventual domain owner will not know a-priori which
device will go to which location.
The bootstrapping process can take minutes to complete depending on
the network infrastructure and device processing speed. The network
communication itself is not optimized for speed; for privacy reasons,
the discovery process allows for the pledge to avoid announcing its
presence through broadcasting.
Nomadic or mobile devices often need to aquire credentials to access
the network at the new location. An example of this is mobile phone
roaming among network operators, or even between cell towers. This
is usually called handoff. BRSKI does not provide a low-latency
handoff which is usually a requirement in such situations. For these
solutions BRSKI can be used to create a relationship (an LDevID) with
the "home" domain owner. The resulting credentials are then used to
provide credentials more appropriate for a low-latency handoff.
1.3.2. Constrained environments
Questions have been posed as to whether this solution is suitable in
general for Internet of Things (IoT) networks. This depends on the
capabilities of the devices in question. The terminology of
[RFC7228] is best used to describe the boundaries.
The solution described in this document is aimed in general at non-
constrained (i.e., class 2+) devices operating on a non-Challenged
network. The entire solution as described here is not intended to be
useable as-is by constrained devices operating on challenged networks
(such as 802.15.4 LLNs).
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Specifically, there are protocol aspects described here that might
result in congestion collapse or energy-exhaustion of intermediate
battery powered routers in an LLN. Those types of networks SHOULD
NOT use this solution. These limitations are predominately related
to the large credential and key sizes required for device
authentication. Defining symmetric key techniques that meet the
operational requirements is out-of-scope but the underlying protocol
operations (TLS handshake and signing structures) have sufficient
algorithm agility to support such techniques when defined.
The imprint protocol described here could, however, be used by non-
energy constrained devices joining a non-constrained network (for
instance, smart light bulbs are usually mains powered, and speak
802.11). It could also be used by non-constrained devices across a
non-energy constrained, but challenged network (such as 802.15.4).
The certificate contents, and the process by which the four questions
above are resolved do apply to constrained devices. It is simply the
actual on-the-wire imprint protocol that could be inappropriate.
1.3.3. Network Access Controls
This document presumes that network access control has either already
occurred, is not required, or is integrated by the proxy and
registrar in such a way that the device itself does not need to be
aware of the details. Although the use of an X.509 Initial Device
Identity is consistant with IEEE 802.1AR [IDevID], and allows for
alignment with 802.1X network access control methods, its use here is
for pledge authentication rather than network access control.
Integrating this protocol with network access control, perhaps as an
Extensible Authentication Protocol (EAP) method (see [RFC3748]), is
out-of-scope.
1.3.4. Bootstrapping is not Booting
This document describes "bootstrapping" as the protocol used to
obtain a local trust anchor. It is expected that this trust anchor,
along with any additional configuration information subsequently
installed, is persisted on the device across system restarts
("booting"). Bootstrapping occurs only infrequently such as when a
device is transfered to a new owner or has been reset to factory
default settings.
1.4. Leveraging the new key infrastructure / next steps
As a result of the protocol described herein, the bootstrapped
devices have the Domain CA trust anchor in common. An end entity
certificate has optionally been issued from the Domain CA. This
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makes it possible to securely deploy functionalities across the
domain, e.g:
o Device management.
o Routing authentication.
o Service discovery.
The major beneficiary is that it possible to use the credentials
deployed by this protocol to secure the Autonomic Control Plane (ACP)
([I-D.ietf-anima-autonomic-control-plane]).
1.5. Requirements for Autonomic Network Infrastructure (ANI) devices
The BRSKI protocol can be used in a number of environments. Some of
the options in this document is the result of requirements that are
out of the ANI scope. This section defines the base requirements for
ANI devices.
For devices that intend to become part of an Autonomic Network
Infrastructure (ANI) ([I-D.ietf-anima-reference-model]) that includes
an Autonomic Control Plane
([I-D.ietf-anima-autonomic-control-plane]), the BRSKI protocol MUST
be implemented.
The pledge must perform discovery of the proxy as described in
Section 4.1 using GRASP M_FLOOD announcements.
Upon successfully validating a voucher artiface, a status telemetry
MUST be returned. See Section 5.7.
An ANIMA ANI pledge MUST implement the EST automation extensions
described in Section 5.9. They supplement the [RFC7030] EST to
better support automated devices that do not have an end user.
The ANI Join Registrar ASA MUST support all the BRSKI and above
listed EST operations.
All ANI devices SHOULD support the BRSKI proxy function, using
circuit proxies over the ACP. (See Section 4.3)
2. Architectural Overview
The logical elements of the bootstrapping framework are described in
this section. Figure 1 provides a simplified overview of the
components.
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2. Identify itself. This is done by presenting an X.509 IDevID
credential to the discovered registrar (via the proxy) in a TLS
handshake. (The registrar credentials are only provisionally
accepted at this time).
3. Request to join the discovered registrar. A unique nonce is
included ensuring that any responses can be associated with this
particular bootstrapping attempt.
4. Imprint on the registrar. This requires verification of the
manufacturer service provided voucher. A voucher contains
sufficient information for the pledge to complete authentication
of a registrar. This document details this step in depth.
5. Enroll. After imprint an authenticated TLS (HTTPS) connection
exists between pledge and registrar. Enrollment over Secure
Transport (EST) [RFC7030] is then used to obtain a domain
certificate from a registrar.
The pledge is now a member of, and can be managed by, the domain and
will only repeat the discovery aspects of bootstrapping if it is
returned to factory default settings.
This specification details integration with EST enrollment so that
pledges can optionally obtain a locally issued certificate, although
any REST interface could be integrated in future work.
2.2. Secure Imprinting using Vouchers
A voucher is a cryptographically protected artifact (a digital
signature) to the pledge device authorizing a zero-touch imprint on
the registrar domain.
The format and cryptographic mechanism of vouchers is described in
detail in [RFC8366].
Vouchers provide a flexible mechanism to secure imprinting: the
pledge device only imprints when a voucher can be validated. At the
lowest security levels the MASA can indiscriminately issue vouchers
and log claims of ownership by domains. At the highest security
levels issuance of vouchers can be integrated with complex sales
channel integrations that are beyond the scope of this document. The
sales channel integration would verify actual (legal) ownership of
the pledge by the domain. This provides the flexibility for a number
of use cases via a single common protocol mechanism on the pledge and
registrar devices that are to be widely deployed in the field. The
MASA services have the flexibility to leverage either the currently
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defined claim mechanisms or to experiment with higher or lower
security levels.
Vouchers provide a signed but non-encrypted communication channel
among the pledge, the MASA, and the registrar. The registrar
maintains control over the transport and policy decisions allowing
the local security policy of the domain network to be enforced.
2.3. Initial Device Identifier
Pledge authentication and pledge voucher-request signing is via a
PKIX certificate installed during the manufacturing process. This is
the 802.1AR Initial Device Identifier (IDevID), and it provides a
basis for authenticating the pledge during the protocol exchanges
described here. There is no requirement for a common root PKI
hierarchy. Each device manufacturer can generate its own root
certificate. Specifically, the IDevID enables:
1. Uniquely identifying the pledge by the Distinguished Name (DN)
and subjectAltName (SAN) parameters in the IDevID. The unique
identification of a pledge in the voucher objects are derived
from those parameters as described below.
2. Provides a cryptographic authentication of the pledge to the
Registrar (see Section 5.3).
3. Secure auto-discovery of the pledge's MASA by the registrar (see
Section 2.8).
4. Signing of voucher-request by the pledge's IDevID (see
Section 3).
5. Provides a cryptographic authentication of the pledge to the MASA
(see Section 5.5.5).
Section 7.2.13 of [IDevID] discusses keyUsage and extendedKeyUsage
extensions in the IDevID certificate. Any restrictions included
reduce the utility of the IDevID and so this specification RECOMMENDS
that no key usage restrictions be included. Additionally, [RFC5280]
section 4.2.1.3 does not require key usage restrictions for end
entity certificates.
2.3.1. Identification of the Pledge
In the context of BRSKI, pledges are uniquely identified by a
"serial-number". This serial-number is used both in the "serial-
number" field of voucher or voucher-requests (see Section 3) and in
local policies on registrar or MASA (see Section 5).
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The following fields are defined in [IDevID] and [RFC5280]:
o The subject field's DN encoding MUST include the "serialNumber"
attribute with the device's unique serial number. (from [IDevID]
section 7.2.8, and [RFC5280] section 4.1.2.4's list of standard
attributes)
o The subject-alt field's encoding MAY include a non-critical
version of the RFC4108 defined HardwareModuleName. (from [IDevID]
section 7.2.9) If the IDevID is stored in a Trusted Platform
Module (TPM), then this field MAY contain the TPM identification
rather than the device's serial number. If both fields are
present, then the subject field takes precedence.
and they are used as follows by the pledge to build the "serial-
number" that is placed in the voucher-request. In order to build it,
the fields need to be converted into a serial-number of "type
string". The following methods are used depending on the first
available IDevID certificate field (attempted in this order):
1. [RFC4519] section 2.31 provides an example ("WI-3005") of the
Distinguished Name "serialNumber" attribute. [RFC4514] indicates
this is a printable string so no encoding is necessary.
2. The HardwareModuleName hwSerialNum OCTET STRING. This value is
base64 encoded to convert it to a printable string format.
The above process to locate the serial-number MUST be performed by
the pledge when filling out the voucher-request. Signed voucher-
requests are always passed up to the MASA.
As explained in Section 5.5 the Registrar MUST extract the serial-
number again itself from the pledge's TLS certificate. It can
consult the serial-number in the pledge-request if there are any
possible confusion about the source of the serial-number (hwSerialNum
vs serialNumber).
2.3.2. MASA URI extension
This docucment defines a new PKIX non-critical certificate extension
to carry the MASA URI. This extension is intended to be used in the
IDevID certificate. The URI is represented as described in
Section 7.4 of [RFC5280].
Any Internationalized Resource Identifiers (IRIs) MUST be mapped to
URIs as specified in Section 3.1 of [RFC3987] before they are placed
in the certificate extension. The IRI provides the authority
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information. The BRSKI "/.well-known" tree ([RFC5785]) is described
in Section 5.
As explained in [RFC5280] section 7.4, a complete IRI SHOULD be in
this extension, including the scheme, iauthority, and ipath. As a
consideration to constrained systems, this MAY be reduced to only the
iauthority, in which case a scheme of "https://" and ipath of
"/.well-known/est" is to be assumed, as explained in section
Section 5.
The registrary can assume that only the iauthority is present in the
extension, if there are no slash ("/") characters in the extension.
Section 7.4 of [RFC5280] calls out various schemes that MUST be
supported, including ldap, http and ftp. However, the registrar MUST
use https for the BRSKI-MASA connection.
The new extension is identified as follows:
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The pledge is the device that is attempting to join. Until the
pledge completes the enrollment process, it has link-local network
connectivity only to the proxy.
2.5.2. Join Proxy
The join proxy provides HTTPS connectivity between the pledge and the
registrar. A circuit proxy mechanism is described in Section 4.
Additional mechanisms, including a CoAP mechanism and a stateless
IPIP mechanism are the subject of future work.
2.5.3. Domain Registrar
The domain's registrar operates as the BRSKI-MASA client when
requesting vouchers from the MASA (see Section 5.4). The registrar
operates as the BRSKI-EST server when pledges request vouchers (see
Section 5.1). The registrar operates as the BRSKI-EST server
"Registration Authority" if the pledge requests an end entity
certificate over the BRSKI-EST connection (see Section 5.9).
The registrar uses an Implicit Trust Anchor database for
authenticating the BRSKI-MASA TLS connection MASA certificate. The
registrar uses a different Implicit Trust Anchor database for
authenticating the BRSKI-EST TLS connection pledge client
certificate. Configuration or distribution of these trust anchor
databases is out-of-scope of this specification.
2.5.4. Manufacturer Service
The Manufacturer Service provides two logically seperate functions:
the Manufacturer Authorized Signing Authority (MASA) described in
Section 5.5 and Section 5.6, and an ownership tracking/auditing
function described in Section 5.7 and Section 5.8.
2.5.5. Public Key Infrastructure (PKI)
The Public Key Infrastructure (PKI) administers certificates for the
domain of concerns, providing the trust anchor(s) for it and allowing
enrollment of pledges with domain certificates.
The voucher provides a method for the distribution of a single PKI
trust anchor (as the "pinned-domain-cert"). A distribution of the
full set of current trust anchors is possible using the optional EST
integration.
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The domain's registrar acts as an [RFC5272] Registration Authority,
requesting certificates for pledges from the Key Infrastructure.
The expectations of the PKI are unchanged from EST [[RFC7030]]. This
document does not place any additional architectural requirements on
the Public Key Infrastructure.
2.6. Certificate Time Validation2.6.1. Lack of realtime clock
Many devices when bootstrapping do not have knowledge of the current
time. Mechanisms such as Network Time Protocols cannot be secured
until bootstrapping is complete. Therefore bootstrapping is defined
in a method that does not require knowledge of the current time. A
pledge MAY ignore all time stamps in the voucher and in the
certificate validity periods if it does not know the current time.
The pledge is exposed to dates in the following five places:
registrar certificate notBefore, registrar certificiate notAfter,
voucher created-on, and voucher expires-on. Additionally, CMS
signatures contain a signingTime.
If the voucher contains a nonce then the pledge MUST confirm the
nonce matches the original pledge voucher-request. This ensures the
voucher is fresh. See Section 5.2.
2.6.2. Infinite Lifetime of IDevID
[RFC5280] explains that long lived pledge certificates "SHOULD be
assigned the GeneralizedTime value of 99991231235959Z". Registrars
MUST support such lifetimes and SHOULD support ignoring pledge
lifetimes if they did not follow the RFC5280 recommendations.
For example, IDevID may have incorrect lifetime of N <= 3 years,
rendering replacement pledges from storage useless after N years
unless registrars support ignoring such a lifetime.
2.7. Cloud Registrar
There exist operationally open network wherein devices gain
unauthenticated access to the internet at large. In these use cases
the management domain for the device needs to be discovered within
the larger internet. These are less likely within the anima scope
but may be more important in the future.
There are additionally some greenfield situations involving an
entirely new installation where a device may have some kind of
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management uplink that it can use (such as via 3G network for
instance). In such a future situation, the device might use this
management interface to learn that it should configure itself to
become the local registrar.
In order to support these scenarios, the pledge MAY contact a well
known URI of a cloud registrar if a local registrar cannot be
discovered or if the pledge's target use cases do not include a local
registrar.
If the pledge uses a well known URI for contacting a cloud registrar
an Implicit Trust Anchor database (see [RFC7030]) MUST be used to
authenticate service as described in [RFC6125]. This is consistent
with the human user configuration of an EST server URI in [RFC7030]
which also depends on RFC6125.
2.8. Determining the MASA to contact
The registrar needs to be able to contact a MASA that is trusted by
the pledge in order to obtain vouchers. There are three mechanisms
described:
The device's Initial Device Identifier (IDevID) will normally contain
the MASA URL as detailed in Section 2.3. This is the RECOMMENDED
mechanism.
If the registrar is integrated with [I-D.ietf-opsawg-mud] and the
pledge IDevID contains the id-pe-mud-url then the registrar MAY
attempt to obtain the MASA URL from the MUD file. The MUD file
extension for the MASA URL is defined in Appendix C.
It can be operationally difficult to ensure the necessary X.509
extensions are in the pledge's IDevID due to the difficulty of
aligning current pledge manufacturing with software releases and
development. As a final fallback the registrar MAY be manually
configured or distributed with a MASA URL for each manufacturer.
Note that the registrar can only select the configured MASA URL based
on the trust anchor -- so manufacturers can only leverage this
approach if they ensure a single MASA URL works for all pledge's
associated with each trust anchor.
3. Voucher-Request artifact
Voucher-requests are how vouchers are requested. The semantics of
the vouchers are described below, in the YANG model.
A pledge forms the "pledge voucher-request" and submits it to the
registrar.
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The registrar in turn forms the "registrar voucher-request", and
submits it to the MASA.
The "proximity-registrar-cert" leaf is used in the pledge voucher-
requests. This provides a method for the pledge to assert the
registrar's proximity.
The "prior-signed-voucher-request" leaf is used in registrar voucher-
requests. If present, it is the signed pledge voucher-request. This
provides a method for the registrar to forward the pledge's signed
request to the MASA. This completes transmission of the signed
"proximity-registrar-cert" leaf.
Unless otherwise signaled (outside the voucher-request artifact), the
signing structure is as defined for vouchers, see [RFC8366].
3.1. Nonceless Voucher Requests
A registrar MAY also retrieve nonceless vouchers by sending nonceless
voucher-requests to the MASA in order to obtain vouchers for use when
the registrar does not have connectivity to the MASA. No "prior-
signed-voucher-request" leaf would be included. The registrar will
also need to know the serial number of the pledge. This document
does not provide a mechanism for the registrar to learn that in an
automated fashion. Typically this will be done via scanning of bar-
code or QR-code on packaging, or via some sales channel integration.
3.2. Tree Diagram
The following tree diagram illustrates a high-level view of a
voucher-request document. The voucher-request builds upon the
voucher artifact described in [RFC8366]. The tree diagram is
described in [RFC8340]. Each node in the diagram is fully described
by the YANG module in Section 3.4. Please review the YANG module for
a detailed description of the voucher-request format.
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{
"ietf-voucher-request:voucher": {
"nonce": "62a2e7693d82fcda2624de58fb6722e5",
"created-on": "2017-01-01T00:00:02.000Z",
"idevid-issuer": "base64encodedvalue=="
"serial-number": "JADA123456789"
"prior-signed-voucher-request": "base64encodedvalue=="
}
}
Example (3) The following example illustrates a registrar voucher-
request. The 'prior-signed-voucher-request' leaf is not
populated with the pledge's voucher-request nor is the
nonce leaf. This form might be used by a registrar
requesting a voucher when the pledge can not communicate
with the registrar (such as when it is powered down, or
still in packaging), and therefore could not submit a
nonce. This scenario is most useful when the registrar
is aware that it will not be able to reach the MASA
during deployment. See Section 5.5.
{
"ietf-voucher-request:voucher": {
"created-on": "2017-01-01T00:00:02.000Z",
"idevid-issuer": "base64encodedvalue=="
"serial-number": "JADA123456789"
}
}
3.4. YANG Module
Following is a YANG [RFC7950] module formally extending the [RFC8366]
voucher into a voucher-request.
<CODE BEGINS> file "ietf-voucher-request@2018-02-14.yang"
module ietf-voucher-request {
yang-version 1.1;
namespace
"urn:ietf:params:xml:ns:yang:ietf-voucher-request";
prefix "vch";
import ietf-restconf {
prefix rc;
description "This import statement is only present to access
the yang-data extension defined in RFC 8040.";
reference "RFC 8040: RESTCONF Protocol";
}
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import ietf-voucher {
prefix v;
description "This module defines the format for a voucher,
which is produced by a pledge's manufacturer or
delegate (MASA) to securely assign a pledge to
an 'owner', so that the pledge may establish a secure
conn ection to the owner's network infrastructure";
reference "RFC YYYY: Voucher Profile for Bootstrapping Protocols";
}
organization
"IETF ANIMA Working Group";
contact
"WG Web: <http://tools.ietf.org/wg/anima/>
WG List: <mailto:anima@ietf.org>
Author: Kent Watsen
<mailto:kwatsen@juniper.net>
Author: Max Pritikin
<mailto:pritikin@cisco.com>
Author: Michael Richardson
<mailto:mcr+ietf@sandelman.ca>
Author: Toerless Eckert
<mailto:tte+ietf@cs.fau.de>";
description
"This module defines the format for a voucher request.
It is a superset of the voucher itself.
It provides content to the MASA for consideration
during a voucher request.
The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT',
'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and 'OPTIONAL' in
the module text are to be interpreted as described in RFC 2119.
Copyright (c) 2017 IETF Trust and the persons identified as
authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, is permitted pursuant to, and subject to the license
terms contained in, the Simplified BSD License set forth in Section4.c of the IETF Trust's Legal Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info).
This version of this YANG module is part of RFC XXXX; see the RFC
itself for full legal notices.";
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included in this field.
For example, a pledge might sign a voucher request
with a proximity-registrar-cert, and the registrar
then includes it in the prior-signed-voucher-request field.
This is a simple mechanism for a chain of trusted
parties to change a voucher request, while
maintaining the prior signature information.
The Registrar and MASA MAY examine the prior signed
voucher information for the
purposes of policy decisions. For example this information
could be useful to a MASA to determine that both pledge and
registrar agree on proximity assertions. The MASA SHOULD
remove all prior-signed-voucher-request information when
signing a voucher for imprinting so as to minimize the
final voucher size.";
}
leaf proximity-registrar-cert {
type binary;
description
"An X.509 v3 certificate structure as specified by RFC 5280,
Section 4 encoded using the ASN.1 distinguished encoding
rules (DER), as specified in ITU-T X.690.
The first certificate in the Registrar TLS server
certificate_list sequence (see [RFC5246]) presented by
the Registrar to the Pledge. This MUST be populated in a
Pledge's voucher request if a proximity assertion is
requested.";
}
}
}
}
}
<CODE ENDS>
4. Proxying details (Pledge - Proxy - Registrar)
The role of the proxy is to facilitate communications. The proxy
forwards packets between the pledge and a registrar that has been
provisioned to the proxy via GRASP discovery.
This section defines a stateful proxy mechanism which is refered to
as a "circuit" proxy.
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The proxy does not terminate the TLS handshake: it passes streams of
bytes onward without examination. A proxy MUST NOT assume any
specific TLS version.
A Registrar can directly provide the proxy announcements described
below, in which case the announced port can point directly to the
Registrar itself. In this scenario the pledge is unaware that there
is no proxing occuring. This is useful for Registrars servicing
pledges on directly connected networks.
As a result of the proxy Discovery process in Section 4.1.1, the port
number exposed by the proxy does not need to be well known, or
require an IANA allocation.
During the discovery of the Registrar by the Join Proxy, the Join
Proxy will also learn which kinds of proxy mechanisms are available.
This will allow the Join Proxy to use the lowest impact mechanism
which the Join Proxy and Registrar have in common.
In order to permit the proxy functionality to be implemented on the
maximum variety of devices the chosen mechanism SHOULD use the
minimum amount of state on the proxy device. While many devices in
the ANIMA target space will be rather large routers, the proxy
function is likely to be implemented in the control plane CPU of such
a device, with available capabilities for the proxy function similar
to many class 2 IoT devices.
The document [I-D.richardson-anima-state-for-joinrouter] provides a
more extensive analysis and background of the alternative proxy
methods.
4.1. Pledge discovery of Proxy
The result of discovery is a logical communication with a registrar,
through a proxy. The proxy is transparent to the pledge. The
communication between the pledge is over IPv6 Link-Local addresses.
To discover the proxy the pledge performs the following actions:
1. MUST: Obtains a local address using IPv6 methods as described in
[RFC4862] IPv6 Stateless Address AutoConfiguration. Use of
[RFC4941] temporary addresses is encouraged. To limit pervasive
monitoring ( [RFC7258]), a new temporary address MAY use a short
lifetime (that is, set TEMP_PREFERRED_LIFETIME to be short).
Pledges will generally prefer use of IPv6 Link-Local addresses,
and discovery of proxy will be by Link-Local mechanisms. IPv4
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2. MUST: Listen for GRASP M_FLOOD ([I-D.ietf-anima-grasp])
announcements of the objective: "AN_Proxy". See section
Section 4.1.1 for the details of the objective. The pledge MAY
listen concurrently for other sources of information, see
Appendix B.
Once a proxy is discovered the pledge communicates with a registrar
through the proxy using the bootstrapping protocol defined in
Section 5.
While the GRASP M_FLOOD mechanism is passive for the pledge, the
optional other methods (mDNS, and IPv4 methods) are active. The
pledge SHOULD run those methods in parallel with listening to for the
M_FLOOD. The active methods SHOULD exponentially back-off to a
maximum of one hour to avoid overloading the network with discovery
attempts. Detection of change of physical link status (ethernet
carrier for instance) SHOULD reset the exponential back off.
The pledge could discover more than one proxy on a given physical
interface. The pledge can have a multitude of physical interfaces as
well: a layer-2/3 ethernet switch may have hundreds of physical
ports.
Each possible proxy offer SHOULD be attempted up to the point where a
voucher is received: while there are many ways in which the attempt
may fail, it does not succeed until the voucher has been validated.
The connection attempts via a single proxy SHOULD exponentially back-
off to a maximum of one hour to avoid overloading the network
infrastructure. The back-off timer for each MUST be independent of
other connection attempts.
Connection attempts SHOULD be run in parallel to avoid head of queue
problems wherein an attacker running a fake proxy or registrar could
perform protocol actions intentionally slowly. The pledge SHOULD
continue to listen to for additional GRASP M_FLOOD messages during
the connection attempts.
Once a connection to a registrar is established (e.g. establishment
of a TLS session key) there are expectations of more timely
responses, see Section 5.2.
Once all discovered services are attempted (assuming that none
succeeded) the device MUST return to listening for GRASP M_FLOOD. It
SHOULD periodically retry the manufacturer specific mechanisms. The
pledge MAY prioritize selection order as appropriate for the
anticipated environment.
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The use of CoAP to connect from pledge to registrar is out of scope
for this document, and is described in future work. See
[I-D.ietf-anima-constrained-voucher].
4.3. Proxy discovery and communication of Registrar
The registrar SHOULD announce itself so that proxies can find it and
determine what kind of connections can be terminated.
The registrar announces itself using ACP instance of GRASP using
M_FLOOD messages. ANI proxies MUST support GRASP discovery of
registrars.
The M_FLOOD is formatted as follows:
[M_FLOOD, 12340815, h'fda379a6f6ee00000200000064000001', 180000,
["AN_join_registrar", 4, 255, "EST-TLS"],
[O_IPv6_LOCATOR,
h'fda379a6f6ee00000200000064000001', IPPROTO_TCP, 80]]
Figure 7a: Registrar Discovery
The formal CDDL definition is:
flood-message = [M_FLOOD, session-id, initiator, ttl,
+[objective, (locator-option / [])]]
objective = ["AN_join_registrar", objective-flags, loop-count,
objective-value]
initiator = ACP address to contact Registrar
objective-flags = sync-only ; as in GRASP spec
sync-only = 4 ; M_FLOOD only requires synchronization
loop-count = 255 ; mandatory maximum
objective-value = text ; name of the (list of) of supported
; protocols: "EST-TLS" for RFC7030.
Figure 7: AN_join_registrar CDDL
The M_FLOOD message MUST be sent periodically. The period is subject
to network administrator policy (EST server configuration). It must
be sufficiently low that the aggregate amount of periodic M_FLOODs
from all EST servers causes negligible traffic across the ACP.
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Here are some examples of locators for illustrative purposes. Only
the first one ($transport-protocol = 6, TCP) is defined in this
document and is mandatory to implement.
locator1 = [O_IPv6_LOCATOR, fd45:1345::6789, 6, 443]
locator2 = [O_IPv6_LOCATOR, fd45:1345::6789, 17, 5683]
locator3 = [O_IPv6_LOCATOR, fe80::1234, 41, nil]
A protocol of 6 indicates that TCP proxying on the indicated port is
desired.
Registrars MUST announce the set of protocols that they support.
They MUST support TCP traffic.
Registrars MUST accept HTTPS/EST traffic on the TCP ports indicated.
Registrars MUST support ANI TLS circuit proxy and therefore BRSKI
across HTTPS/TLS native across the ACP.
In the ANI, the Autonomic Control Plane (ACP) secured instance of
GRASP ([I-D.ietf-anima-grasp]) MUST be used for discovery of ANI
registrar ACP addresses and ports by ANI proxies. The TCP leg of the
proxy connection between ANI proxy and ANI registrar therefore also
runs across the ACP.
5. Protocol Details (Pledge - Registrar - MASA)
The pledge MUST initiate BRSKI after boot if it is unconfigured. The
pledge MUST NOT automatically initiate BRSKI if it has been
configured or is in the process of being configured.
BRSKI is described as extensions to EST [RFC7030]. The goal of these
extensions is to reduce the number of TLS connections and crypto
operations required on the pledge. The registrar implements the
BRSKI REST interface within the same "/.well-known" URI tree as the
existing EST URIs as described in EST [RFC7030] section 3.2.2. The
communication channel between the pledge and the registrar is
referred to as "BRSKI-EST" (see Figure 1).
The communication channel between the registrar and MASA is similarly
described as extensions to EST within the same "/.well-known" tree.
For clarity this channel is referred to as "BRSKI-MASA". (See
Figure 1).
MASA URI is "https://" iauthority "/.well-known/est".
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BRSKI uses existing CMS message formats for existing EST operations.
BRSKI uses JSON [RFC7159] for all new operations defined here, and
voucher formats.
While EST section 3.2 does not insist upon use of HTTP 1.1 persistent
connections, BRSKI-EST connections SHOULD use persistent connections.
The intention of this guidance is to ensure the provisional TLS state
occurs only once, and that the subsequent resolution of the provision
state is not subject to a MITM attack during a critical phase.
Summarized automation extensions for the BRSKI-EST flow are:
o The pledge either attempts concurrent connections via each
discovered proxy, or it times out quickly and tries connections in
series, as explained at the end of Section 5.1.
o The pledge provisionally accepts the registrar certificate during
the TLS handshake as detailed in Section 5.1.
o The pledge requests and validates a voucher using the new REST
calls described below.
o The pledge completes authentication of the server certificate as
detailed in Section 5.6.1. This moves the BRSKI-EST TLS
connection out of the provisional state.
o Mandatory boostrap steps conclude with voucher status telemetry
(see Section 5.7).
The BRSKI-EST TLS connection can now be used for EST enrollment.
The extensions for a registrar (equivalent to EST server) are:
o Client authentication is automated using Initial Device Identity
(IDevID) as per the EST certificate based client authentication.
The subject field's DN encoding MUST include the "serialNumber"
attribute with the device's unique serial number.
o In the language of [RFC6125] this provides for a SERIALNUM-ID
category of identifier that can be included in a certificate and
therefore that can also be used for matching purposes. The
SERIALNUM-ID whitelist is collated according to manufacturer trust
anchor since serial numbers are not globally unique.
o The registrar requests and validates the voucher from the MASA.
o The registrar forwards the voucher to the pledge when requested.
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o The registrar performs log verifications in addition to local
authorization checks before accepting optional pledge device
enrollment requests.
5.1. BRSKI-EST TLS establishment details
The pledge establishes the TLS connection with the registrar through
the circuit proxy (see Section 4) but the TLS handshake is with the
registrar. The BRSKI-EST pledge is the TLS client and the BRSKI-EST
registrar is the TLS server. All security associations established
are between the pledge and the registrar regardless of proxy
operations.
Establishment of the BRSKI-EST TLS connection is as specified in EST
[RFC7030] section 4.1.1 "Bootstrap Distribution of CA Certificates"
[RFC7030] wherein the client is authenticated with the IDevID
certificate, and the EST server (the registrar) is provisionally
authenticated with an unverified server certificate.
The pledge maintains a security paranoia concerning the provisional
state, and all data received, until a voucher is received and
verified as specified in Section 5.6.1
A Pledge that can connect to multiple registries concurrently, SHOULD
do so. Some devices may be unable to do so for lack of threading, or
resource issues. Concurrent connections defeat atttempts by a
malicious proxy from causing a TCP Slowloris-like attack (see
[slowloris]).
A pledge that can not maintain as many connections as there are
eligible proxies. If no connection is making process after 5 seconds
then the pledge SHOULD drop the oldest connection and go on to a
different proxy: the proxy that has been communicated with least
recently. If there were no other proxies discovered, the pledge MAY
continue to wait, as long as it is concurrently listening for new
proxy announcements.
5.2. Pledge Requests Voucher from the Registrar
When the pledge bootstraps it makes a request for a voucher from a
registrar.
This is done with an HTTPS POST using the operation path value of
"/.well-known/est/requestvoucher".
The pledge voucher-request Content-Type is:
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application/voucher-cms+json The request is a "YANG-defined JSON
document that has been signed using a CMS structure" as described
in Section 3 using the JSON encoding described in [RFC7951]. This
voucher media type is defined in [RFC8366] and is also used for
the pledge voucher-request. The pledge SHOULD sign the request
using the Section 2.3 credential.
Registrar impementations SHOULD anticipate future media types but of
course will simply fail the request if those types are not yet known.
The pledge SHOULD include an [RFC7231] section 5.3.2 "Accept" header
indicating the acceptable media type for the voucher response. The
"application/voucher-cms+json" media type is defined in [RFC8366] but
constrained voucher formats are expected in the future. Registrar's
and MASA's are expected to be flexible in what they accept.
The pledge populates the voucher-request fields as follows:
created-on: Pledges that have a realtime clock are RECOMMENDED to
populate this field. This provides additional information to the
MASA.
nonce: The pledge voucher-request MUST contain a cryptographically
strong random or pseudo-random number nonce. (see [RFC4086]) Doing
so ensures Section 2.6.1 functionality. The nonce MUST NOT be
reused for multiple bootstrapping attempts. (The registrar
voucher-request MAY omit the nonce as per Section 3.1)
proximity-registrar-cert: In a pledge voucher-request this is the
first certificate in the TLS server 'certificate_list' sequence
(see [RFC5246]) presented by the registrar to the pledge. This
MUST be populated in a pledge voucher-request if the "proximity"
assertion is populated.
All other fields MAY be omitted in the pledge voucher-request.
An example JSON payload of a pledge voucher-request is in Section 3.3
Example 1.
The registrar validates the client identity as described in EST
[RFC7030] section 3.3.2. The registrar confirms that the 'proximity'
assertion and associated 'proximity-registrar-cert' are correct.
5.3. Registrar Authorization of Pledge
In a fully automated network all devices must be securely identified
and authorized to join the domain.
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A Registrar accepts or declines a request to join the domain, based
on the authenticated identity presented. Automated acceptance
criteria include:
o allow any device of a specific type (as determined by the X.509
IDevID),
o allow any device from a specific vendor (as determined by the
X.509 IDevID),
o allow a specific device from a vendor (as determined by the X.509
IDevID) against a domain white list. (The mechanism for checking
a shared white list potentially used by multiple Registrars is out
of scope).
If these validations fail the registrar SHOULD respond with an
appropriate HTTP error code.
If authorization is successful the registrar obtains a voucher from
the MASA service (see Section 5.5) and returns that MASA signed
voucher to the pledge as described in Section 5.6.
5.4. BRSKI-MASA TLS establishment details
The BRSKI-MASA TLS connection is a 'normal' TLS connection
appropriate for HTTPS REST interfaces. The registrar initiates the
connection and uses the MASA URL obtained as described in Section 2.8
for [RFC6125] authentication of the MASA.
The primary method of registrar "authentication" by the MASA is
detailed in Section 5.5. As detailed in Section 11 the MASA might
find it necessary to request additional registrar authentication.
The MASA and the registrars SHOULD be prepared to support TLS client
certificate authentication and/or HTTP Basic or Digest authentication
as described in [RFC7030] for EST clients. This connection MAY also
have no client authentication at all (Section 7.4)
The authentication of the BRSKI-MASA connection does not affect the
voucher-request process, as voucher-requests are already signed by
the registrar. Instead, this authentication provides access control
to the audit log.
Implementors are advised that contacting the MASA is to establish a
secured REST connection with a web service and that there are a
number of authentication models being explored within the industry.
Registrars are RECOMMENDED to fail gracefully and generate useful
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administrative notifications or logs in the advent of unexpected HTTP
401 (Unauthorized) responses from the MASA.
5.5. Registrar Requests Voucher from MASA
When a registrar receives a pledge voucher-request it in turn submits
a registrar voucher-request to the MASA service via an HTTPS RESTful
interface ([RFC7231]).
This is done with an HTTP POST using the operation path value of
"/.well-known/est/requestvoucher".
The voucher media type "application/voucher-cms+json" is defined in
[RFC8366] and is also used for the registrar voucher-request. It is
a JSON document that has been signed using a CMS structure. The
registrar MUST sign the registrar voucher-request. The entire
registrar certificate chain, up to and including the Domain CA, MUST
be included in the CMS structure.
MASA impementations SHOULD anticipate future media types but of
course will simply fail the request if those types are not yet known.
The Registrar SHOULD include an [RFC7231] section 5.3.2 "Accept"
header indicating the response media types that are acceptable. This
list SHOULD be the entire list presented to the Registrar in the
Pledge's original request (see Section 5.2) but MAY be a subset.
MASA's are expected to be flexible in what they accept.
The registrar populates the voucher-request fields as follows:
created-on: Registrars are RECOMMENDED to populate this field. This
provides additional information to the MASA.
nonce: This is the value from the pledge voucher-request. The
registrar voucher-request MAY omit the nonce as per Section 3.1)
serial-number: The serial number of the pledge the registrar would
like a voucher for. The registrar determines this value by
parsing the authenticated pledge IDevID certificate. See
Section 2.3. The registrar MUST verify that the serial number
field it parsed matches the serial number field the pledge
provided in its voucher-request. This provides a sanity check
useful for detecting error conditions and logging. The registrar
MUST NOT simply copy the serial number field from a pledge voucher
request as that field is claimed but not certified.
idevid-issuer: The idevid-issuer value from the pledge certificate
is included to ensure a statistically unique identity.
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prior-signed-voucher-request: The signed pledge voucher-request
SHOULD be included in the registrar voucher-request. (NOTE: what
is included is the complete pledge voucher-request, inclusive of
the 'assertion', 'proximity-registrar-cert', etc wrapped by the
pledge's original signature). If a signed voucher-request was not
recieved from the pledge then this leaf is omitted from the
registrar voucher request.
A nonceless registrar voucher-request MAY be submitted to the MASA.
Doing so allows the registrar to request a voucher when the pledge is
offline, or when the registrar anticipates not being able to connect
to the MASA while the pledge is being deployed. Some use cases
require the registrar to learn the appropriate IDevID SerialNumber
field and appropriate 'Accept header' field values from the physical
device labeling or from the sales channel (out-of-scope for this
document).
All other fields MAY be omitted in the registrar voucher-request.
Example JSON payloads of registrar voucher-requests are in
Section 3.3 Examples 2 through 4.
The MASA verifies that the registrar voucher-request is internally
consistent but does not necessarily authenticate the registrar
certificate since the registrar is not known to the MASA in advance.
The MASA performs the actions and validation checks described in the
following sub-sections before issuing a voucher.
5.5.1. MASA renewal of expired vouchers
As described in [RFC8366] vouchers are normally short lived to avoid
revocation issues. If the request is for a previous (expired)
voucher using the same registrar then the request for a renewed
voucher SHOULD be automatically authorized. The MASA has sufficient
information to determine this by examining the request, the registrar
authentication, and the existing audit log. The issuance of a
renewed voucher is logged as detailed in Section 5.6.
To inform the MASA that existing vouchers are not to be renewed one
can update or revoke the registrar credentials used to authorize the
request (see Section 5.5.3 and Section 5.5.4). More flexible methods
will likely involve sales channel integration and authorizations
(details are out-of-scope of this document).
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The MASA MUST verify that the registrar voucher-request is signed by
a registrar. This is confirmed by verifying that the id-kp-cmcRA
extended key usage extension field (as detailed in EST RFC7030
section 3.6.1) exists in the certificate of the entity that signed
the registrar voucher-request. This verification is only a
consistency check that the unauthenticated domain CA intended the
voucher-request signer to be a registrar. Performing this check
provides value to the domain PKI by assuring the domain administrator
that the MASA service will only respect claims from authorized
Registration Authorities of the domain.
The MASA verifies that the domain CA certificate is included in the
CMS structure as detailed in Section 5.5.
5.5.3. MASA authentication of registrar (certificate)
If a nonceless voucher-request is submitted the MASA MUST
authenticate the registrar as described in either EST [RFC7030]
section 3.2, section 3.3, or by validating the registrar's
certificate used to sign the registrar voucher-request. Any of these
methods reduce the risk of DDoS attacks and provide an authenticated
identity as an input to sales channel integration and authorizations
(details are out-of-scope of this document).
In the nonced case, validation of the registrar MAY be omitted if the
device policy is to accept audit-only vouchers.
5.5.4. MASA revocation checking of registrar (certificate)
As noted in Section 5.5.3 the MASA performs registrar authentication
in a subset of situations (e.g. nonceless voucher requests). Normal
PKIX revocation checking is assumed during either EST client
authentication or voucher-request signature validation. Similarly,
as noted in Section 5.5.2, the MASA performs normal PKIX revocation
checking during signature consistency checks (a signature by a
registrar certificate that has been revoked is an inconsistency).
5.5.5. MASA verification of pledge prior-signed-voucher-request
The MASA MAY verify that the registrar voucher-request includes the
'prior-signed-voucher-request' field. If so the prior-signed-
voucher-request MUST include a 'proximity-registrar-cert' that is
consistent with the certificate used to sign the registrar voucher-
request. Additionally the voucher-request serial-number leaf MUST
match the pledge serial-number that the MASA extracts from the
signing certificate of the prior-signed-voucher-request. The MASA is
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aware of which pledges support signing of their voucher requests and
can use this information to confirm proximity of the pledge with the
registrar, thus ensuring that the BRSKI-EST TLS connection has no
man-in-the-middle.
If these checks succeed the MASA updates the voucher and audit log
assertion leafs with the "proximity" assertion.
5.5.6. MASA pinning of registrar
The registrar's certificate chain is extracted from the signature
method. The chain includes the domain CA certificate as specified in
Section 5.5. This certificate is used to populate the "pinned-
domain-cert" of the voucher being issued. The domainID (e.g., hash
of the root public key) is determined from the pinned-domain-cert and
is used to update the audit log.
5.5.7. MASA nonce handling
The MASA does not verify the nonce itself. If the registrar voucher-
request contains a nonce, and the prior-signed-voucher-request is
exist, then the MASA MUST verify that the nonce is consistent.
(Recall from above that the voucher-request might not contain a
nonce, see Section 5.5 and Section 5.5.3).
The MASA MUST use the nonce from the registrar voucher-request for
the resulting voucher and audit log. The prior-signed-voucher-
request nonce is ignored during this operation.
5.6. MASA and Registrar Voucher Response
The MASA voucher response to the registrar is forwarded without
changes to the pledge; therefore this section applies to both the
MASA and the registrar. The HTTP signaling described applies to both
the MASA and registrar responses. A registrar either caches prior
MASA responses or dynamically requests a new voucher based on local
policy (it does not generate or sign a voucher). Registrar
evaluation of the voucher itself is purely for transparency and audit
purposes to further inform log verification (see Section 5.8.2) and
therefore a registrar could accept future voucher formats that are
opaque to the registrar.
If the voucher-request is successful, the server (MASA responding to
registrar or registrar responding to pledge) response MUST contain an
HTTP 200 response code. The server MUST answer with a suitable 4xx
or 5xx HTTP [RFC2616] error code when a problem occurs. In this
case, the response data from the MASA MUST be a plaintext human-
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readable (ASCII, English) error message containing explanatory
information describing why the request was rejected.
The registrar MAY respond with an HTTP 202 ("the request has been
accepted for processing, but the processing has not been completed")
as described in EST [RFC7030] section 4.2.3 wherein the client "MUST
wait at least the specified 'Retry-After' time before repeating the
same request". (see [RFC7231] section 6.6.4) The pledge is
RECOMMENDED to provide local feedback (blinked LED etc) during this
wait cycle if mechanisms for this are available. To prevent an
attacker registrar from significantly delaying bootstrapping the
pledge MUST limit the 'Retry-After' time to 60 seconds. Ideally the
pledge would keep track of the appropriate Retry-After header values
for any number of outstanding registrars but this would involve a
state table on the pledge. Instead the pledge MAY ignore the exact
Retry-After value in favor of a single hard coded value (a registrar
that is unable to complete the transaction after the first 60 seconds
has another chance a minute later). A pledge SHOULD only maintain a
202 retry-state for up to 4 days, which is longer than a long
weekend, after which time the enrollment attempt fails and the pledge
returns to discovery state.
In order to avoid infinite redirect loops, which a malicious
registrar might do in order to keep the pledge from discovering the
correct registrar, the pledge MUST NOT follow more than one
redirection (3xx code) to another web origins. EST supports
redirection but requires user input; this change allows the pledge to
follow a single redirection without a user interaction.
A 403 (Forbidden) response is appropriate if the voucher-request is
not signed correctly, stale, or if the pledge has another outstanding
voucher that cannot be overridden.
A 404 (Not Found) response is appropriate when the request is for a
device that is not known to the MASA.
A 406 (Not Acceptable) response is appropriate if a voucher of the
desired type or using the desired algorithms (as indicated by the
Accept: headers, and algorithms used in the signature) cannot be
issued such as because the MASA knows the pledge cannot process that
type. The registrar SHOULD use this response if it determines the
pledge is unacceptable due to inventory control, MASA audit logs, or
any other reason.
A 415 (Unsupported Media Type) response is approriate for a request
that has a voucher-request or accept encoding that is not understood.
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The voucher response format is as indicated in the submitted accept
header or based on the MASA's prior understanding of proper format
for this Pledge. Only the [RFC8366] "application/voucher-cms+json"
media type is defined at this time. The syntactic details of
vouchers are described in detail in [RFC8366]. For example, the
voucher consists of:
{
"ietf-voucher:voucher": {
"nonce": "62a2e7693d82fcda2624de58fb6722e5",
"assertion": "logging"
"pinned-domain-cert": "base64encodedvalue=="
"serial-number": "JADA123456789"
}
}
The MASA populates the voucher fields as follows:
nonce: The nonce from the pledge if available. See Section 5.5.7.
assertion: The method used to verify assertion. See Section 5.5.5.
pinned-domain-cert: The domain CA cert. See Section 5.5.6. This
figure is illustrative, for an example, see Appendix D.2
serial-number: The serial-number as provided in the voucher-request.
Also see Section 5.5.5.
domain-cert-revocation-checks: Set as appropriate for the pledge's
capabilities and as documented in [RFC8366]. The MASA MAY set
this field to 'false' since setting it to 'true' would require
that revocation information be available to the pledge and this
document does not make normative requirements for [RFC6961] or
equivalent integrations.
expires-on: This is set for nonceless vouchers. The MASA ensures
the voucher lifetime is consistent with any revocation or pinned-
domain-cert consistency checks the pledge might perform. See
section Section 2.6.1. There are three times to consider: (a) a
configured voucher lifetime in the MASA, (b) the expiry time for
the registrar's certificate, (c) any certificate revocation
information (CRL) lifetime. The expires-on field SHOULD be before
the earliest of these three values. Typically (b) will be some
significant time in the future, but (c) will typically be short
(on the order of a week or less). The RECOMMENDED period for (a)
is on the order of 20 minutes, so it will typically determine the
lifespan of the resulting voucher. 20 minutes is sufficent time
to reach the post-provisional state in the pledge, at which point
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there is an established trust relationship between pledge and
registrar. The subsequent operations can take as long as required
from that point onwards. The lifetime of the voucher has no
impact on the lifespan of the ownership relationship.
Whenever a voucher is issued the MASA MUST update the audit log
appropriately. The internal state requirements to maintain the audit
log are out-of-scope. See Section 5.8.1 for a discussion of
reporting the log to a registrar.
5.6.1. Pledge voucher verification
The pledge MUST verify the voucher signature using the manufacturer
installed trust anchor(s) associated with the manufacturer's MASA
(this is likely included in the pledge's firmware). Management of
the manufacter installed trust anchor(s) is out-of-scope of this
document; this protocol does not update these trust anchor(s).
The pledge MUST verify the serial-number field of the signed voucher
matches the pledge's own serial-number.
The pledge MUST verify that the voucher nonce field is accurate and
matches the nonce the pledge submitted to this registrar, or that the
voucher is nonceless (see Section 7.2).
The pledge MUST be prepared to parse and fail gracefully from a
voucher response that does not contain a 'pinned-domain-cert' field.
The pledge MUST be prepared to ignore additional fields that it does
not recognize.
5.6.2. Pledge authentication of provisional TLS connection
The 'pinned-domain-cert' element of the voucher contains the domain
CA's public key. The pledge MUST use the 'pinned-domain-cert' trust
anchor to immediately complete authentication of the provisional TLS
connection.
If a registrar's credentials cannot be verified using the pinned-
domain-cert trust anchor from the voucher then the TLS connection is
immediately discarded and the pledge abandons attempts to bootstrap
with this discovered registrar. The pledge SHOULD send voucher
status telemetry (described below) before closing the TLS connection.
The pledge MUST attempt to enroll using any other proxies it has
found. It SHOULD return to the same proxy again after attempting
with other proxies. Attempts should be attempted in the exponential
backoff described earlier. Attempts SHOULD be repeated as failure
may be the result of a temporary inconsistently (an inconsistently
rolled registrar key, or some other mis-configuration). The
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inconsistently could also be the result an active MITM attack on the
EST connection.
The registrar MUST use a certificate that chains to the pinned-
domain-cert as its TLS server certificate.
The pledge's PKIX path validation of a registrar certificate's
validity period information is as described in Section 2.6.1. Once
the PKIX path validation is successful the TLS connection is no
longer provisional.
The pinned-domain-cert MAY be installed as an trust anchor for future
operations such as enrollment (e.g. [RFC7030] as recommended) or
trust anchor management or raw protocols that do not need full PKI
based key management. It can be used to authenticate any dynamically
discovered EST server that contain the id-kp-cmcRA extended key usage
extension as detailed in EST RFC7030 section 3.6.1; but to reduce
system complexity the pledge SHOULD avoid additional discovery
operations. Instead the pledge SHOULD communicate directly with the
registrar as the EST server. The 'pinned-domain-cert' is not a
complete distribution of the [RFC7030] section 4.1.3 CA Certificate
Response, which is an additional justification for the recommendation
to proceed with EST key management operations. Once a full CA
Certificate Response is obtained it is more authoritative for the
domain than the limited 'pinned-domain-cert' response.
5.7. Pledge BRSKI Status Telemetry
The domain is expected to provide indications to the system
administrators concerning device lifecycle status. To facilitate
this it needs telemetry information concerning the device's status.
To indicate pledge status regarding the voucher, the pledge MUST post
a status message.
The posted data media type: application/json
The client HTTP POSTs the following to the server at the EST well
known URI "/voucher_status". The Status field indicates if the
voucher was acceptable. If it was not acceptable the Reason string
indicates why. In the failure case this message may be sent to an
unauthenticated, potentially malicious registrar and therefore the
Reason string SHOULD NOT provide information beneficial to an
attacker. The operational benefit of this telemetry information is
balanced against the operational costs of not recording that an
voucher was ignored by a client the registrar expected to continue
joining the domain.
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{
"version":"1",
"Status":FALSE /* TRUE=Success, FALSE=Fail"
"Reason":"Informative human readable message"
"reason-context": { additional JSON }
}
The server SHOULD respond with an HTTP 200 but MAY simply fail with
an HTTP 404 error. The client ignores any response. Within the
server logs the server SHOULD capture this telemetry information.
The reason-context attribute is an arbitrary JSON object (literal
value or hash of values) which provides additional information
specific to this pledge. The contents of this field are not subject
to standardization.
Additional standard JSON fields in this POST MAY be added, see
Section 8.3.
5.8. Registrar audit log request
After receiving the pledge status telemetry Section 5.7, the
registrar SHOULD request the MASA audit log from the MASA service.
This is done with an HTTP GET using the operation path value of
"/.well-known/est/requestauditlog".
The registrar SHOULD HTTP POST the same registrar voucher-request as
it did when requesting a voucher (using the same Content-Type). It
is posted to the /requestauditlog URI instead. The "idevid-issuer"
and "serial-number" informs the MASA which log is requested so the
appropriate log can be prepared for the response. Using the same
media type and message minimizes cryptographic and message operations
although it results in additional network traffic. The relying MASA
implementation MAY leverage internal state to associate this request
with the original, and by now already validated, voucher-request so
as to avoid an extra crypto validation.
A registrar MAY request logs at future times. If the registrar
generates a new request then the MASA is forced to perform the
additional cryptographic operations to verify the new request.
A MASA that receives a request for a device that does not exist, or
for which the requesting owner was never an owner returns an HTTP 404
("Not found") code.
Rather than returning the audit log as a response to the POST (with a
return code 200), the MASA MAY instead return a 201 ("Created")
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RESTful response ([RFC7231] section 7.1) containing a URL to the
prepared (and easily cachable) audit response.
In order to avoid enumeration of device audit logs, MASA that return
URLs SHOULD take care to make the returned URL unguessable. For
instance, rather than returning URLs containing a database number
such as https://example.com/auditlog/1234 or the EUI of the device
such https://example.com/auditlog/10-00-00-11-22-33, the MASA SHOULD
return a randomly generated value (a "slug" in web parlance). The
value is used to find the relevant database entry.
A MASA that returns a code 200 MAY also include a Location: header
for future reference by the registrar.
5.8.1. MASA audit log response
A log data file is returned consisting of all log entries associated
with the the device selected by the IDevID presented in the request.
The audit log may be truncated of old or repeated values as explained
below. The returned data is in JSON format ([RFC7951]), and the
Content-Type SHOULD be "application/json". For example:
{
"version":"1",
"events":[
{
"date":"<date/time of the entry>",
"domainID":"<domainID extracted from voucher-request>",
"nonce":"<any nonce if supplied (or the exact string 'NULL')>"
"assertion":"<the value from the voucher assertion leaf>"
"truncated":"<the number of domainID entries truncated>"
},
{
"date":"<date/time of the entry>",
"domainID":"<anotherDomainID extracted from voucher-request>",
"nonce":"<any nonce if supplied (or the exact string 'NULL')>"
"assertion":"<the value from the voucher assertion leaf>"
}
],
"truncation": {
"nonced duplicates": "<total number of entries truncated>",
"nonceless duplicates": "<total number of entries truncated>",
"arbitrary": "<number of domainID entries removed entirely>"
}
}
Distribution of a large log is less than ideal. This structure can
be optimized as follows: Nonced or Nonceless entries for the same
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domainID MAY be truncated from the log leaving only the single most
recent nonced or nonceless entry for that domainID. In the case of
truncation the 'event' truncation value SHOULD contain a count of the
number of events for this domainID that were truncated. The log
SHOULD NOT be further reduced but there could exist operational
situation where maintaining the full log is not possible. In such
situations the log MAY be arbitrarily truncated for length, with the
number of removed entries indicated as 'arbitrary'.
If the truncation count exceeds 1024 then the MASA MAY use this value
without further incrementing it.
A log where duplicate entries for the same domain have been truncated
("nonced duplicates" and/or "nonceless duplicates) could still be
acceptable for informed decisions. A log that has had "arbitrary"
truncations is less acceptable but manufacturer transparency is
better than hidden truncations.
This document specifies a simple log format as provided by the MASA
service to the registrar. This format could be improved by
distributed consensus technologies that integrate vouchers with
technologies such as block-chain or hash trees or optimized logging
approaches. Doing so is out of the scope of this document but is an
anticipated improvement for future work. As such, the registrar
client SHOULD anticipate new kinds of responses, and SHOULD provide
operator controls to indicate how to process unknown responses.
5.8.2. Registrar audit log verification
Each time the Manufacturer Authorized Signing Authority (MASA) issues
a voucher, it places it into the audit log for that device. The
details are described in Section 5.8. The contents of the audit log
can express a variety of trust levels, and this section explains what
kind of trust a registrar can derive from the entries.
While the audit log provides a list of vouchers that were issued by
the MASA, the vouchers are issued in response to voucher-requests,
and it is the contents of the voucher-requests which determines how
meaningful the audit log entries are.
A registrar SHOULD use the log information to make an informed
decision regarding the continued bootstrapping of the pledge. The
exact policy is out of scope of this document as it depends on the
security requirements within the registrar domain. Equipment that is
purchased pre-owned can be expected to have an extensive history.
The following dicussion is provided to help explain the value of each
log element:
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date: The date field provides the registrar an opportunity to divide
the log around known events such as the purchase date. Depending
on context known to the registrar or administrator evens before/
after certain dates can have different levels of importance. For
example for equipment that is expected to be new, and thus have no
history, it would be a surprise to find prior entries.
domainID: If the log includes an unexpected domainID then the pledge
could have imprinted on an unexpected domain. The registrar can
be expected to use a variety of techniques to define "unexpected"
ranging from white lists of prior domains to anomoly detection
(e.g. "this device was previously bound to a different domain than
any other device deployed"). Log entries can also be compared
against local history logs in search of discrepancies (e.g. "this
device was re-deployed some number of times internally but the
external audit log shows additional re-deployments our internal
logs are unaware of").
nonce: Nonceless entries mean the logged domainID could
theoretically trigger a reset of the pledge and then take over
management by using the existing nonceless voucher.
assertion: The assertion leaf in the voucher and audit log indicates
why the MASA issued the voucher. A "verified" entry means that
the MASA issued the associated voucher as a result of positive
verification of ownership but this can still be problematic for
registrar's that expected only new (not pre-owned) pledges. A
"logged" assertion informs the registrar that the prior vouchers
were issued with minimal verification. A "proximity" assertion
assures the registrar that the pledge was truly communicating with
the prior domain and thus provides assurance that the prior domain
really has deployed the pledge.
A relatively simple policy is to white list known (internal or
external) domainIDs and to require all vouchers to have a nonce and/
or require that all nonceless vouchers be from a subset (e.g. only
internal) domainIDs. A simple action is to revoke any locally issued
credentials for the pledge in question or to refuse to forward the
voucher. A registrar MAY be configured to ignore the history of the
device but it is RECOMMENDED that this only be configured if hardware
assisted NEA [RFC5209] is supported.
5.9. EST Integration for PKI bootstrapping
The pledge SHOULD follow the BRSKI operations with EST enrollment
operations including "CA Certificates Request", "CSR Attributes" and
"Client Certificate Request" or "Server-Side Key Generation", etc.
This is a relatively seamless integration since BRSKI REST calls
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provide an automated alternative to the manual bootstrapping method
described in [RFC7030]. As noted above, use of HTTP 1.1 persistent
connections simplifies the pledge state machine.
Although EST allows clients to obtain multiple certificates by
sending multiple CSR requests BRSKI mandates use of the CSR
Attributes request and mandates that the registrar validate the CSR
against the expected attributes. This implies that client requests
will "look the same" and therefore result in a single logical
certificate being issued even if the client were to make multiple
requests. Registrars MAY contain more complex logic but doing so is
out-of-scope of this specification. BRSKI does not signal any
enhancement or restriction to this capability.
5.9.1. EST Distribution of CA Certificates
The pledge SHOULD request the full EST Distribution of CA
Certificates message. See RFC7030, section 4.1.
This ensures that the pledge has the complete set of current CA
certificates beyond the pinned-domain-cert (see Section 5.6.1 for a
discussion of the limitations inherent in having a single certificate
instead of a full CA Certificates response.) Although these
limitations are acceptable during initial bootstrapping, they are not
appropriate for ongoing PKIX end entity certificate validation.
5.9.2. EST CSR Attributes
Automated bootstrapping occurs without local administrative
configuration of the pledge. In some deployments it is plausible
that the pledge generates a certificate request containing only
identity information known to the pledge (essentially the X.509
IDevID information) and ultimately receives a certificate containing
domain specific identity information. Conceptually the CA has
complete control over all fields issued in the end entity
certificate. Realistically this is operationally difficult with the
current status of PKI certificate authority deployments, where the
CSR is submitted to the CA via a number of non-standard protocols.
Even with all standardized protocols used, it could operationally be
problematic to expect that service specific certificate fields can be
created by a CA that is likely operated by a group that has no
insight into different network services/protocols used. For example,
the CA could even be outsourced.
To alleviate these operational difficulties, the pledge MUST request
the EST "CSR Attributes" from the EST server and the EST server needs
to be able to reply with the attributes necessary for use of the
certificate in its intended protocols/services. This approach allows
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for minimal CA integrations and instead the local infrastructure (EST
server) informs the pledge of the proper fields to include in the
generated CSR. This approach is beneficial to automated boostrapping
in the widest number of environments.
If the hardwareModuleName in the X.509 IDevID is populated then it
SHOULD by default be propagated to the LDevID along with the
hwSerialNum. The EST server SHOULD support local policy concerning
this functionality.
In networks using the BRSKI enrolled certificate to authenticate the
ACP (Autonomic Control Plane), the EST attributes MUST include the
"ACP information" field. See
[I-D.ietf-anima-autonomic-control-plane] for more details.
The registrar MUST also confirm that the resulting CSR is formatted
as indicated before forwarding the request to a CA. If the registrar
is communicating with the CA using a protocol such as full CMC, which
provides mechanisms to override the CSR attributes, then these
mechanisms MAY be used even if the client ignores CSR Attribute
guidance.
5.9.3. EST Client Certificate Request
The pledge MUST request a new client certificate. See RFC7030,
section 4.2.
5.9.4. Enrollment Status Telemetry
For automated bootstrapping of devices, the adminstrative elements
providing bootstrapping also provide indications to the system
administrators concerning device lifecycle status. This might
include information concerning attempted bootstrapping messages seen
by the client, MASA provides logs and status of credential
enrollment. [RFC7030] assumes an end user and therefore does not
include a final success indication back to the server. This is
insufficient for automated use cases.
To indicate successful enrollment the client SHOULD re-negotiate the
EST TLS session using the newly obtained credentials. This occurs by
the client initiating a new TLS ClientHello message on the existing
TLS connection. The client MAY simply close the old TLS session and
start a new one. The server MUST support either model.
In the case of a FAIL, the Reason string indicates why the most
recent enrollment failed. The SubjectKeyIdentifier field MUST be
included if the enrollment attempt was for a keypair that is locally
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known to the client. If EST /serverkeygen was used and failed then
the field is omitted from the status telemetry.
In the case of a SUCCESS the Reason string is omitted. The
SubjectKeyIdentifier is included so that the server can record the
successful certificate distribution.
Status media type: application/json
The client HTTP POSTs the following to the server at the new EST well
known URI /enrollstatus.
{
"version":"1",
"Status":TRUE /* TRUE=Success, FALSE=Fail"
"Reason":"Informative human readable message"
"reason-context": "Additional information"
}
The server SHOULD respond with an HTTP 200 but MAY simply fail with
an HTTP 404 error.
Within the server logs the server MUST capture if this message was
received over an TLS session with a matching client certificate.
This allows for clients that wish to minimize their crypto operations
to simply POST this response without renegotiating the TLS session -
at the cost of the server not being able to accurately verify that
enrollment was truly successful.
5.9.5. Multiple certificates
Pledges that require multiple certificates could establish direct EST
connections to the registrar.
5.9.6. EST over CoAP
This document describes extensions to EST for the purposes of
bootstrapping of remote key infrastructures. Bootstrapping is
relevant for CoAP enrollment discussions as well. The defintion of
EST and BRSKI over CoAP is not discussed within this document beyond
ensuring proxy support for CoAP operations. Instead it is
anticipated that a definition of CoAP mappings will occur in
subsequent documents such as [I-D.ietf-ace-coap-est] and that CoAP
mappings for BRSKI will be discussed either there or in future work.
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[RFC7030] defines it's endpoints to include a "Content-Transfer-
Encoding" heading, and the payloads to be [RFC4648] Base64 encoded
DER.
When used within BRSKI, the original RFC7030 EST endpoints remain
Base64 encoded, but the new BRSKI end points which send and receive
binary artifacts (specifically, ../voucherrequest) are binary. That
is, no encoding is used.
In the BRSKI context, the EST "Content-Transfer-Encoding" header if
present, SHOULD be ignored. This header does not need to included.
7. Reduced security operational modes
A common requirement of bootstrapping is to support less secure
operational modes for support specific use cases. The following
sections detail specific ways that the pledge, registrar and MASA can
be configured to run in a less secure mode for the indicated reasons.
This section is considered non-normative: use suggested methods MUST
be detailed in specific profiles of BRSKI. This is the subject for
future work.
7.1. Trust Model
This section explains the trust relationships detailed in
Section 2.4:
+--------+ +---------+ +------------+ +------------+
| Pledge | | Join | | Domain | |Manufacturer|
| | | Proxy | | Registrar | | Service |
| | | | | | | (Internet) |
+--------+ +---------+ +------------+ +------------+
Figure 10
Pledge: The pledge could be compromised and providing an attack
vector for malware. The entity is trusted to only imprint using
secure methods described in this document. Additional endpoint
assessment techniques are RECOMMENDED but are out-of-scope of this
document.
Join Proxy: Provides proxy functionalities but is not involved in
security considerations.
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Registrar: When interacting with a MASA a registrar makes all
decisions. For Ownership Audit Vouchers (see [RFC8366]) the
registrar is provided an opportunity to accept MASA decisions.
Vendor Service, MASA: This form of manufacturer service is trusted
to accurately log all claim attempts and to provide authoritative
log information to registrars. The MASA does not know which
devices are associated with which domains. These claims could be
strengthened by using cryptographic log techniques to provide
append only, cryptographic assured, publicly auditable logs.
Current text provides only for a trusted manufacturer.
Vendor Service, Ownership Validation: This form of manufacturer
service is trusted to accurately know which device is owned by
which domain.
7.2. Pledge security reductions
The pledge can choose to accept vouchers using less secure methods.
These methods enable offline and emergency (touch based) deployment
use cases:
1. The pledge MUST accept nonceless vouchers. This allows for a use
case where the registrar can not connect to the MASA at the
deployment time. Logging and validity periods address the
security considerations of supporting these use cases.
2. Many devices already support "trust on first use" for physical
interfaces such as console ports. This document does not change
that reality. Devices supporting this protocol MUST NOT support
"trust on first use" on network interfaces. This is because
"trust on first use" over network interfaces would undermine the
logging based security protections provided by this
specification.
3. The pledge MAY have an operational mode where it skips voucher
validation one time. For example if a physical button is
depressed during the bootstrapping operation. This can be useful
if the manufacturer service is unavailable. This behavior SHOULD
be available via local configuration or physical presence methods
(such as use of a serial/craft console) to ensure new entities
can always be deployed even when autonomic methods fail. This
allows for unsecured imprint.
It is RECOMMENDED that "trust on first use" or any method of skipping
voucher validation (including use of craft serial console) only be
available if hardware assisted Network Endpoint Assessment [RFC5209]
is supported. This recommendation ensures that domain network
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monitoring can detect innappropriate use of offline or emergency
deployment procedures when voucher-based bootstrapping is not used.
7.3. Registrar security reductions
A registrar can choose to accept devices using less secure methods.
These methods are acceptable when low security models are needed, as
the security decisions are being made by the local administrator, but
they MUST NOT be the default behavior:
1. A registrar MAY choose to accept all devices, or all devices of a
particular type, at the administrator's discretion. This could
occur when informing all registrars of unique identifiers of new
entities might be operationally difficult.
2. A registrar MAY choose to accept devices that claim a unique
identity without the benefit of authenticating that claimed
identity. This could occur when the pledge does not include an
X.509 IDevID factory installed credential. New Entities without
an X.509 IDevID credential MAY form the Section 5.2 request using
the Section 5.5 format to ensure the pledge's serial number
information is provided to the registrar (this includes the
IDevID AuthorityKeyIdentifier value, which would be statically
configured on the pledge.) The pledge MAY refuse to provide a
TLS client certificate (as one is not available.) The pledge
SHOULD support HTTP-based or certificate-less TLS authentication
as described in EST RFC7030 section 3.3.2. A registrar MUST NOT
accept unauthenticated New Entities unless it has been configured
to do so by an administrator that has verified that only expected
new entities can communicate with a registrar (presumably via a
physically secured perimeter.)
3. A registrar MAY submit a nonceless voucher-requests to the MASA
service (by not including a nonce in the voucher-request.) The
resulting vouchers can then be stored by the registrar until they
are needed during bootstrapping operations. This is for use
cases where the target network is protected by an air gap and
therefore cannot contact the MASA service during pledge
deployment.
4. A registrar MAY ignore unrecognized nonceless log entries. This
could occur when used equipment is purchased with a valid history
being deployed in air gap networks that required permanent
vouchers.
5. A registrar MAY accept voucher formats of future types that can
not be parsed by the Registrar. This reduces the Registrar's
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visibility into the exact voucher contents but does not change
the protocol operations.
7.4. MASA security reductions
Lower security modes chosen by the MASA service affect all device
deployments unless bound to the specific device identities. In which
case these modes can be provided as additional features for specific
customers. The MASA service can choose to run in less secure modes
by:
1. Not enforcing that a nonce is in the voucher. This results in
distribution of a voucher that never expires and in effect makes
the Domain an always trusted entity to the pledge during any
subsequent bootstrapping attempts. That this occurred is
captured in the log information so that the registrar can make
appropriate security decisions when a pledge joins the Domain.
This is useful to support use cases where registrars might not be
online during actual device deployment. Because this results in
a long lived voucher and does not require the proof that the
device is online, this is only accepted when the registrar is
authenticated by the MASA and authorized to provide this
functionality. The MASA is RECOMMENDED to use this functionality
only in concert with an enhanced level of ownership tracking
(out-of-scope.) If the pledge device is known to have a real-
time-clock that is set from the factory, use of a voucher
validity period is RECOMMENDED.
2. Not verifying ownership before responding with a voucher. This
is expected to be a common operational model because doing so
relieves the manufacturer providing MASA services from having to
track ownership during shipping and supply chain and allows for a
very low overhead MASA service. A registrar uses the audit log
information as a defense in depth strategy to ensure that this
does not occur unexpectedly (for example when purchasing new
equipment the registrar would throw an error if any audit log
information is reported.) The MASA SHOULD verify the 'prior-
signed-voucher-request' information for pledges that support that
functionality. This provides a proof-of-proximity check that
reduces the need for ownership verification.
8. IANA Considerations
This document requires the following IANA actions:
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This document extends the definitions of "est" (so far defined via
RFC7030) in the "https://www.iana.org/assignments/well-known-uris/well-known-uris.xhtml" registry as follows:
o add /.well-known/est/requestvoucher (see Section 5.5 )
o add /.well-known/est/requestauditlog (see Section 5.7)
8.2. PKIX Registry
IANA is requested to register the following:
This document requests a number for id-mod-MASAURLExtn2016(TBD) from
the pkix(7) id-mod(0) Registry.
This document has received an early allocation from the id-pe
registry (SMI Security for PKIX Certificate Extension) for id-pe-
masa-url with the value 32, resulting in an OID of
1.3.6.1.5.5.7.1.32.
8.3. Pledge BRSKI Status Telemetry
IANA is requested to create a new Registry entitled: "BRSKI
Parameters", and within that Registry to create a table called:
"Pledge BRSKI Status Telemetry Attributes". New items can be added
using the Specification Required. The following items are to be in
the initial registration, with this document (Section 5.7) as the
reference:
o version
o Status
o Reason
o reason-context
8.4. DNS Service Names
IANA is requested to register the following Service Names:
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Service Name: _brski-proxy
Transport Protocol(s): tcp
Assignee: IESG <iesg@ietf.org>.
Contact: IESG <iesg@ietf.org>
Description: The Bootstrapping Remote Secure Key
Infrastructures Proxy
Reference: [This document]
Service Name: _brski-registrar
Transport Protocol(s): tcp
Assignee: IESG <iesg@ietf.org>.
Contact: IESG <iesg@ietf.org>
Description: The Bootstrapping Remote Secure Key
Infrastructures Registrar
Reference: [This document]
8.5. MUD File Extension for the MASA
The IANA is requested to list the name "masa" in the MUD extensions
registry defined in [I-D.ietf-opsawg-mud]. Its use is documented in
Appendix C.
9. Applicability to the Autonomic Control Plane
This document provides a solution to the requirements for secure
bootstrap set out in Using an Autonomic Control Plane for Stable
Connectivity of Network Operations, Administration, and Maintenance
[RFC8368], A Reference Model for Autonomic Networking
[I-D.ietf-anima-reference-model] and specifically the An Autonomic
Control Plane (ACP) [I-D.ietf-anima-autonomic-control-plane], section3.2 (Secure Bootstrap), and section 6.1 (ACP Domain, Certificate and
Network).
The protocol described in this document has appeal in a number of
other non-ANIMA use cases. Such uses of the protocol will be
deploying into other environments with different tradeoffs of
privacy, security, reliability and autonomy from manufacturers. As
such those use cases will need to provide their own applicability
statements, and will need to address unique privacy and security
considerations for the environments in which they are used.
The autonomic control plane that this document provides bootstrap for
is typically a medium to large Internet Service Provider
organization, or an equivalent Enterprise that has signficant layer-3
router connectivity. (A network consistenting of primarily layer-2
is not excluded, but the adjacencies that the ACP will create and
maintain will not reflect the topology until all devices participate
in the ACP).
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As specified in the ANIMA charter, this work "..focuses on
professionally-managed networks." Such a network has an operator and
can do things like like install, configure and operate the Registrar
function. The operator makes purchasing decisions and is aware of
what manufacturers it expects to see on it's network.
Such an operator also is capable of performing the traditional (craft
serial-console) based bootstrap of devices. The zero-touch mechanism
presented in this and the ACP document represents a signficiant
efficiency: in particular it reduces the need to put senior experts
on airplanes to configure devices in person. There is a recognition
as the technology evolves that not every situation may work out, and
occasionally a human still still have to visit.
The BRSKI protocol is going into environments where there have
already been quite a number of vendor proprietary management systems.
Those are not expected to go away quickly, but rather to leverage the
secure credentials that are provisioned by BRSKI. The connectivity
requirements of said management systems are provided by the ACP.
10. Privacy Considerations10.1. MASA audit log
The MASA audit log includes a hash of the domainID for each Registrar
a voucher has been issued to. This information is closely related to
the actual domain identity, especially when paired with the anti-DDoS
authentication information the MASA might collect. This could
provide sufficient information for the MASA service to build a
detailed understanding the devices that have been provisioned within
a domain.
There are a number of design choices that mitigate this risk. The
domain can maintain some privacy since it has not necessarily been
authenticated and is not authoritatively bound to the supply chain.
Additionally the domainID captures only the unauthenticated subject
key identifier of the domain. A privacy sensitive domain could
theoretically generate a new domainID for each device being deployed.
Similarly a privacy sensitive domain would likely purchase devices
that support proximity assertions from a manufacturer that does not
require sales channel integrations. This would result in a
significant level of privacy while maintaining the security
characteristics provided by Registrar based audit log inspection.
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Internet-Draft BRSKI July 201910.2. What BRSKI-MASA reveals to the manufacturer
The so-called "call-home" mechanism that occurs as part of the BRSKI-
MASA connection standardizes what has been deemed by some as a
sinister mechanism for corporate oversight of individuals.
([livingwithIoT] and [IoTstrangeThings] for a small sample).
As the Autonomic Control Plane (ACP) usage of BRSKI is not targetted
at individual usage of IoT devices, but rather at the Enterprise and
ISP creation of networks in a zero-touch fashion, the "call-home"
represents a different kind of concern.
It needs to be re-iterated that the BRSKI-MASA mechanism only occurs
once during the comissioning of the device. It is well defined, and
although encrypted with TLS, it could in theory be made auditable as
the contents are well defined. This connection does not occur when
the device powers on or is restarted for normal routines. It is
conceivable that a device could be forced to go through a full
factory reset during an exceptional firmware update situation, after
which enrollment would have be repeated.
The BRSKI call-home mechanism is mediated via the owner's Registrar,
and the information that is transmitted is directly auditable by the
device owner. This is in stark constrast to many "call-home"
protocols where the device autonomously calls home and uses an
undocumented protocol.
While the contents of the signed part of the pledge voucher request
can not be changed, they are not encrypted at the registrar. The
ability to audit the messages by the owner of the network prevents
exfiltration of data by a nefarious pledge. The contents of an
unsigned voucher request are, however, completely changeable by the
Registrar. Both are, to re-iterate, encrypted by TLS while in
transit.
The BRSKI-MASA exchange reveals the following information to the
manufacturer:
o the identity of the device being enrolled (down to the serial-
number!).
o an identity of the domain owner in the form of the domain trust
anchor. However, this is not a global PKI anchored name within
the WebPKI, so this identity could be pseudonymous. If there is
sales channel integration, then the MASA will have authenticated
the domain owner, either via pinned certificate, or perhaps
another HTTP authentication method, as per Section 5.5.3.
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o the time the device is activated,
o the IP address of the domain Owner's Registrar. For ISPs and
Enterprises, the IP address provides very clear geolocation of the
owner. No amount of IP address privacy extensions ([RFC4941]) can
do anything about this, as a simple whois lookup likely identifies
the ISP or Enterprise from the upper bits anyway. A passive
attacker who observes the connection definitely may conclude that
the given enterprise/ISP is a customer of the particular equipment
vendor. The precise model that is being enrolled will remain
private.
The above situation is to be distinguished from a residential/
individual person who registers a device from a manufacturer: that an
enterprise/ISP purchases routing products is hardly worth mentioning.
Deviations would, however, be notable.
The situation is not improved by the enterprise/ISP using
anonymization services such as ToR [Dingledine2004], as a TLS 1.2
connection will reveal the ClientCertificate used, clearly
identifying the enterprise/ISP involved. TLS 1.3 is better in this
regard, but an active attacker can still discover the parties
involved by performing a Man-In-The-Middle-Attack on the first
attempt (breaking/killing it with a TCP RST), and then letting
subsequent connection pass through.
A manufacturer could attempt to mix the BRSKI-MASA traffic in with
general traffic their site by hosting the MASA behind the same (set)
of load balancers that the companies normal marketing site is hosted
behind. This makes lots of sense from a straight capacity planning
point of view as the same set of services (and the same set of
Distributed Denial of Service mitigations) may be used.
Unfortunately, as the BRSKI-MASA connections include TLS
ClientCertificate exchanges, this may easily be observed in TLS 1.2,
and a traffic analysis may reveal it even in TLS 1.3. This does not
make such a plan irrelevant. There may be other organizational
reasons to keep the marketing site (which is often subject to
frequent redesigs, outsourcing, etc.) seperate from the MASA, which
may need to operate reliably for decades.
10.3. Manufacturers and Used or Stolen Equipment
As explained above, the manufacturer receives information each time
that a device which is in factory-default mode does a zero-touch
bootstrap, and attempts to enroll into a domain owner's registrar.
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The manufacturer is therefore in a position to decline to issue a
voucher if it detects that the new owner is not the same as the
previous owner.
1. This can be seen as a feature if the equipment is believed to
have been stolen. If the legitimate owner notifies the
manufacturer of the theft, then when the new owner brings the
device up, if they use the zero-touch mechanism, the new
(illegitimate) owner reveals their location and identity.
2. In the case of Used equipment, the initial owner could inform the
manufacturer of the sale, or the manufacturer may just permit
resales unless told otherwise. In which case, the transfer of
ownership simply occurs.
3. A manufacturer could however decide not to issue a new voucher in
response to a transfer of ownership. This is essentially the
same as the stolen case, with the manufacturer having decided
that the sale was not legitimate.
4. There is a fourth case, if the manufacturer is providing
protection against stolen devices. The manufacturer then has a
responsability to protect the legitimate owner against fraudulent
claims that the the equipment was stolen. Such a claim would
cause the manufacturer to refuse to issue a new voucher. Should
the device go through a deep factory reset (for instance,
replacement of a damaged main board component, the device would
not bootstrap.
5. Finally, there is a fifth case: the manufacturer has decided to
end-of-line the device, or the owner has not paid a yearly
support amount, and the manufacturer refuses to issue new
vouchers at that point. This last case is not new to the
industry: many license systems are already deployed that have
significantly worse effect.
This section has outlined five situations in which a manufacturer
could use the voucher system to enforce what are clearly license
terms. A manufacturer that attempted to enforce license terms via
vouchers would find it rather ineffective as the terms would only be
enforced when the device is enrolled, and this is not (to repeat), a
daily or even monthly occurrance.
10.4. Manufacturers and Grey market equipment
Manufacturers of devices often sell different products into different
regional markets. Which product is available in which market can be
driven by price differentials, support issues (some markets may
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require manuals and tech-support to be done in the local language),
government export regulation (such as whether strong crypto is
permitted to be exported, or permitted to be used in a particular
market). When an domain owner obtains a device from a different
market (they can be new) and transfers it to a different location,
this is called a Grey Market.
A manufacturer could decide not to issue a voucher to an enterprise/
ISP based upon their location. There are a number of ways which this
could be determined: from the geolocation of the registrar, from
sales channel knowledge about the customer, and what products are
(un-)available in that market. If the device has a GPS the
coordinates of the device could even be placed into an extension of
the voucher.
The above actions are not illegal, and not new. Many manufacturers
have shipped crypto-weak (exportable) versions of firmware as the
default on equipment for decades. The first task of an enterprise/
ISP has always been to login to a manufacturer system, show one's
"entitlement" (country informatin, proof that support payments have
been made), and receive either a new updated firmware, or a license
key that will activate the correct firmware.
BRSKI permits the above process to automated (in an autonomic
fashion), and therefore perhaps encourages this kind of
differentiation by reducing the cost of doing it.
An issue that manufacturers will need to deal with in the above
automated process is when a device is shipped to one country with one
set of rules (or laws or entitlements), but the domain registry is in
another one. Which rules apply is something will have to be worked
out: the manufacturer could come to believe they are dealing with
Grey market equipment, when it is simply dealing with a global
enterprise.
10.5. Some mitigations for meddling by manufacturers
The most obvious mitigation is not to buy the product. Pick
manufacturers that are up-front about their policies, who do not
change them gratutiously.
A manufacturer could provide a mechanism to manage the trust anchors
and built-in certificates (IDevID) as an extension. This is a
substantial amount of work, and may be an area for future
standardization work.
Replacement of the voucher validation anchors (usually pointing to
the original manufacturer's MASA) with those of the new owner permits
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the new owner to issue vouchers to subsequent owners. This would be
done by having the selling (old) owner to run a MASA.
In order to automatically find the new MASA, the mechanism describe
in this document is to look for the MASA URL extension in the IDevID.
A new owner could override this in their Registrar, or the
manufacturer could provide a mechanism to update or replace the
IDevID prior to sale.
Once the voucher trust anchor and the IDevID is replaced, then the
device will no longer trust the manufacturer in any way. When a new
owner performs a bootstrap, the device will point to a MASA that has
been chosen, and will validate vouchers from this new entity.
The BRSKI protocol depends upon a trust anchor on the device and an
identity on the device. Management of these these entities
facilitiates a few new operatonal modes without making any changes to
the BRSKI protocol. Those modes include: offline modes where the
domain owner operates an internal MASA for all devices, resell modes
where the first domain owner becomes the MASA for the next (resold-
to) domain owner, and services where an aggregator acquires a large
variety of devices, and then acts as a pseudonymized MASA for a
variety of devices from a variety of manufacturers.
Some manufacturers may wish to consider replacement of the IDevID as
an indication that the device's warantee is terminated. For others,
the privacy requiments of some deployments might consider this a
standard operating practice.
As discussed at the end of Section 5.8.1, new work could be done to
use a distributed consensus technology for the audit log. This would
permit the audit log to continue to be useful, even when there is a
chain of MASA due to changes of ownership.
11. Security Considerations
This document details a protocol for bootstrapping that balances
operational concerns against security concerns. As detailed in the
introduction, and touched on again in Section 7, the protocol allows
for reduced security modes. These attempt to deliver additional
control to the local administrator and owner in cases where less
security provides operational benefits. This section goes into more
detail about a variety of specific considerations.
To facilitate logging and administrative oversight, in addition to
triggering Registration verification of MASA logs, the pledge reports
on voucher parsing status to the registrar. In the case of a
failure, this information is informative to a potentially malicious
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registrar. This is mandated anyway because of the operational
benefits of an informed administrator in cases where the failure is
indicative of a problem. The registrar is RECOMMENDED to verify MASA
logs if voucher status telemetry is not received.
To facilitate truely limited clients EST RFC7030 section 3.3.2
requirements that the client MUST support a client authentication
model have been reduced in Section 7 to a statement that the
registrar "MAY" choose to accept devices that fail cryptographic
authentication. This reflects current (poor) practices in shipping
devices without a cryptographic identity that are NOT RECOMMENDED.
During the provisional period of the connection the pledge MUST treat
all HTTP header and content data as untrusted data. HTTP libraries
are regularly exposed to non-secured HTTP traffic: mature libraries
should not have any problems.
Pledges might chose to engage in protocol operations with multiple
discovered registrars in parallel. As noted above they will only do
so with distinct nonce values, but the end result could be multiple
vouchers issued from the MASA if all registrars attempt to claim the
device. This is not a failure and the pledge choses whichever
voucher to accept based on internal logic. The registrars verifying
log information will see multiple entries and take this into account
for their analytics purposes.
11.1. DoS against MASA
There are uses cases where the MASA could be unavailable or
uncooperative to the Registrar. They include active DoS attacks,
planned and unplanned network partitions, changes to MASA policy, or
other instances where MASA policy rejects a claim. These introduce
an operational risk to the Registrar owner in that MASA behavior
might limit the ability to bootstrap a pledge device. For example
this might be an issue during disaster recovery. This risk can be
mitigated by Registrars that request and maintain long term copies of
"nonceless" vouchers. In that way they are guaranteed to be able to
bootstrap their devices.
The issuance of nonceless vouchers themselves creates a security
concern. If the Registrar of a previous domain can intercept
protocol communications then it can use a previously issued nonceless
voucher to establish management control of a pledge device even after
having sold it. This risk is mitigated by recording the issuance of
such vouchers in the MASA audit log that is verified by the
subsequent Registrar and by Pledges only bootstrapping when in a
factory default state. This reflects a balance between enabling MASA
independence during future bootstrapping and the security of
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bootstrapping itself. Registrar control over requesting and auditing
nonceless vouchers allows device owners to choose an appropriate
balance.
The MASA is exposed to DoS attacks wherein attackers claim an
unbounded number of devices. Ensuring a registrar is representative
of a valid manufacturer customer, even without validating ownership
of specific pledge devices, helps to mitigate this. Pledge
signatures on the pledge voucher-request, as forwarded by the
registrar in the prior-signed-voucher-request field of the registrar
voucher-request, significantly reduce this risk by ensuring the MASA
can confirm proximity between the pledge and the registrar making the
request. This mechanism is optional to allow for constrained
devices. Supply chain integration ("know your customer") is an
additional step that MASA providers and device vendors can explore.
11.2. Freshness in Voucher-Requests
A concern has been raised that the pledge voucher-request should
contain some content (a nonce) provided by the registrar and/or MASA
in order for those actors to verify that the pledge voucher-request
is fresh.
There are a number of operational problems with getting a nonce from
the MASA to the pledge. It is somewhat easier to collect a random
value from the registrar, but as the registrar is not yet vouched
for, such a registrar nonce has little value. There are privacy and
logistical challenges to addressing these operational issues, so if
such a thing were to be considered, it would have to provide some
clear value. This section examines the impacts of not having a fresh
pledge voucher-request.
Because the registrar authenticates the pledge, a full Man-in-the-
Middle attack is not possible, despite the provisional TLS
authentication by the pledge (see Section 5.) Instead we examine the
case of a fake registrar (Rm) that communicates with the pledge in
parallel or in close time proximity with the intended registrar.
(This scenario is intentionally supported as described in
Section 4.1.)
The fake registrar (Rm) can obtain a voucher signed by the MASA
either directly or through arbitrary intermediaries. Assuming that
the MASA accepts the registrar voucher-request (either because Rm is
collaborating with a legitimate registrar according to supply chain
information, or because the MASA is in audit-log only mode), then a
voucher linking the pledge to the registrar Rm is issued.
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Such a voucher, when passed back to the pledge, would link the pledge
to registrar Rm, and would permit the pledge to end the provisional
state. It now trusts Rm and, if it has any security vulnerabilities
leveragable by an Rm with full administrative control, can be assumed
to be a threat against the intended registrar.
This flow is mitigated by the intended registrar verifying the audit
logs available from the MASA as described in Section 5.8. Rm might
chose to collect a voucher-request but wait until after the intended
registrar completes the authorization process before submitting it.
This pledge voucher-request would be 'stale' in that it has a nonce
that no longer matches the internal state of the pledge. In order to
successfully use any resulting voucher the Rm would need to remove
the stale nonce or anticipate the pledge's future nonce state.
Reducing the possibility of this is why the pledge is mandated to
generate a strong random or pseudo-random number nonce.
Additionally, in order to successfully use the resulting voucher the
Rm would have to attack the pledge and return it to a bootstrapping
enabled state. This would require wiping the pledge of current
configuration and triggering a re-bootstrapping of the pledge. This
is no more likely than simply taking control of the pledge directly
but if this is a consideration the target network is RECOMMENDED to
take the following steps:
o Ongoing network monitoring for unexpected bootstrapping attempts
by pledges.
o Retreival and examination of MASA log information upon the
occurance of any such unexpected events. Rm will be listed in the
logs along with nonce information for analysis.
11.3. Trusting manufacturers
The BRSKI extensions to EST permit a new pledge to be completely
configured with domain specific trust anchors. The link from built-
in manufacturer-provided trust anchors to domain-specific trust
anchors is mediated by the signed voucher artifact.
If the manufacturer's IDevID signing key is not properly validated,
then there is a risk that the network will accept a pledge that
should not be a member of the network. As the address of the
manufacturer's MASA is provided in the IDevID using the extension
from Section 2.3, the malicious pledge will have no problem
collaborating with it's MASA to produce a completely valid voucher.
BRSKI does not, however, fundamentally change the trust model from
domain owner to manufacturer. Assuming that the pledge used its
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Internet-Draft BRSKI July 2019
IDevID with RFC7030 EST and BRSKI, the domain (registrar) still needs
to trust the manufacturer.
Establishing this trust between domain and manufacturer is outside
the scope of BRSKI. There are a number of mechanisms that can
adopted including:
o Manually configuring each manufacturer's trust anchor.
o A Trust-On-First-Use (TOFU) mechanism. A human would be queried
upon seeing a manufacturer's trust anchor for the first time, and
then the trust anchor would be installed to the trusted store.
There are risks with this; even if the key to name is validated
using something like the WebPKI, there remains the possibility
that the name is a look alike: e.g, dem0.example. vs demO.example.
o scanning the trust anchor from a QR code that came with the
packaging (this is really a manual TOFU mechanism)
o some sales integration process where trust anchors are provided as
part of the sales process, probably included in a digital packing
"slip", or a sales invoice.
o consortium membership, where all manufacturers of a particular
device category (e.g, a light bulb, or a cable-modem) are signed
by an certificate authority specifically for this. This is done
by CableLabs today. It is used for authentication and
authorization as part of TR-79: [docsisroot] and [TR069].
The existing WebPKI provides a reasonable anchor between manufacturer
name and public key. It authenticates the key. It does not provide
a reasonable authorization for the manufacturer, so it is not
directly useable on it's own.
11.4. Manufacturer Maintainance of trust anchors
BRSKI depends upon the manufacturer building in trust anchors to the
pledge device. The voucher artifact which is signed by the MASA will
be validated by the pledge using that anchor. This implies that the
manufacturer needs to maintain access to a signing key that the
pledge can validate.
The manufacturer will need to maintain the ability to make signatures
that can be validated for the lifetime that the device could be
onboarded. Whether this onboarding lifetime is less than the device
lifetime depends upon how the device is used. An inventory of
devices kept in a warehouse as spares might not be onboarded for many
decades.
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There are good cryptographic hygiene reasons why a manufacturer would
not want to maintain access to a private key for many decades. A
manufacturer in that situation can leverage a long-term certificate
authority anchor, built-in to the pledge, and then a certificate
chain may be incorporated using the normal CMS certificate set. This
may increase the size of the voucher artifacts, but that is not a
significant issues in non-constrained environements.
There are a few other operational variations that manufacturers could
consider. For instance, there is no reason that every device need
have the same set of trust anchors pre-installed. Devices built in
different factories, or on different days, or any other consideration
could have different trust anchors built in, and the record of which
batch the device is in would be recorded in the asset database. The
manufacturer would then know which anchor to sign an artifact
against.
Aside from the concern about long-term access to private keys, a
major limiting factor for the shelf-life of many devices will be the
age of the cryptographic algorithms included. A device produced in
2019 will have hardware and software capable of validating algorithms
common in 2019, and will have no defense against attacks (both
quantum and von-neuman brute force attacks) which have not yet been
invented. This concern is orthogonal to the concern about access to
private keys, but this concern likely dominates and limits the
lifespan of a device in a warehouse. If any update to firmware to
support new cryptographic mechanism were possible (while the device
was in a warehouse), updates to trust anchors would also be done at
the same time.
12. Acknowledgements
We would like to thank the various reviewers for their input, in
particular William Atwood, Brian Carpenter, Toerless Eckert, Fuyu
Eleven, Eliot Lear, Sergey Kasatkin, Anoop Kumar, Markus Stenberg,
Peter van der Stok, and Thomas Werner
Significant reviews were done by Jari Arko, Christian Huitema and
Russ Housley.
13. References13.1. Normative References
[I-D.ietf-anima-autonomic-control-plane]
Eckert, T., Behringer, M., and S. Bjarnason, "An Autonomic
Control Plane (ACP)", draft-ietf-anima-autonomic-control-plane-19 (work in progress), March 2019.
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Internet-Draft BRSKI July 2019Appendix B. mDNS / DNSSD proxy discovery options
Pledge discovery of the proxy (Section 4.1) MAY be performed with
DNS-based Service Discovery [RFC6763] over Multicast DNS [RFC6762] to
discover the proxy at "_brski-proxy._tcp.local.".
Proxy discovery of the registrar (Section 4.3) MAY be performed with
DNS-based Service Discovery over Multicast DNS to discover registrars
by searching for the service "_brski-registrar._tcp.local.".
To prevent unaccceptable levels of network traffic, when using mDNS,
the congestion avoidance mechanisms specified in [RFC6762] section 7
MUST be followed. The pledge SHOULD listen for an unsolicited
broadcast response as described in [RFC6762]. This allows devices to
avoid announcing their presence via mDNS broadcasts and instead
silently join a network by watching for periodic unsolicited
broadcast responses.
Discovery of registrar MAY also be performed with DNS-based service
discovery by searching for the service "_brski-
registrar._tcp.example.com". In this case the domain "example.com"
is discovered as described in [RFC6763] section 11 (Appendix A.2
suggests the use of DHCP parameters).
If no local proxy or registrar service is located using the GRASP
mechanisms or the above mentioned DNS-based Service Discovery methods
the pledge MAY contact a well known manufacturer provided
bootstrapping server by performing a DNS lookup using a well known
URI such as "brski-registrar.manufacturer.example.com". The details
of the URI are manufacturer specific. Manufacturers that leverage
this method on the pledge are responsible for providing the registrar
service. Also see Section 2.7.
The current DNS services returned during each query are maintained
until bootstrapping is completed. If bootstrapping fails and the
pledge returns to the Discovery state, it picks up where it left off
and continues attempting bootstrapping. For example, if the first
Multicast DNS _bootstrapks._tcp.local response doesn't work then the
second and third responses are tried. If these fail the pledge moves
on to normal DNS-based Service Discovery.
Appendix C. MUD Extension
The following extension augments the MUD model to include a single
node, as described in [I-D.ietf-opsawg-mud] section 3.6, using the
following sample module that has the following tree structure:
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Internet-Draft BRSKI July 2019D.1.4. Pledge key pair
The pledge has an IDevID key pair built in at manufacturing time:
-----BEGIN EC PRIVATE KEY-----
MHcCAQEEIBgR6SV+uEvWfl5zCQWZxWjYbMhXPyNqdHJ3KPh11mm4oAoGCCqGSM49
AwEHoUQDQgAEWi/jqPpRJ0JgWghZRgeZlLKutbXVjmnHb+1AYaEF/YQjE2g5FZV8
KjiR/bkEl+l8M4onIC7KHaXKKkuag9S6Tw==
-----END EC PRIVATE KEY-----
The public key is used by the registrar to find the MASA. The MASA
URL is in an extension described in Section 2.3.
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
The pledge public certificate as decoded by openssl's x509 utility so
that the extensions can be seen. There is a second Custom Extension
is included to provided to contain the EUI48/EUI64 that the pledge
will configure as it's layer-2 address (this is non-normative).
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